Photo  by  the  author. 


Photomicrograph  of  a  small  block  of  western  hemlock.  At  the  top  is  the  cross 
section  showing  to  the  right  the  late  wood  of  one  season's  growth,  to  the  left  the 
early  wood  of  the  next  season.  The  other  two  sections  are  longitudinal  and  show  the 
fibrous  character  of  the  wood.  To  the  left  is  the  radial  section  with  three  rays 
crossing  it.  To  the  right  is  the  tangential  section  upon  which  the  rays  appear  as 
vertical  rows  of  beads.  X  35. 


THE  ^MECHANICAL 
PROPERTIES    OF  WOOD 


Including  a  Discussion  of  the  Factors  Affecting  the 

Mechanical  Properties,  and  Methods 

of  Timber  Testing 


BV     r  X 

SAMUEL  J.  RECORD,  M.A..M.F. 

ASSISTANT   PROFESSOR   OF   FOREST  PRODUCTS, 
YALE   UNIVERSITY 


FIRST  EDITION 
FIRST   THOUSAND 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 

LONDON:  CHAPMAN  &  HALL,  LIMITED 

1914 


Copyright,  1914,  by 
SAMUEL  J.  RECORD 


PUBLISHERS  PRINTING  COMPANY 
207-217  West  Twenty-fifth  Street.  New  York 


TO    THE   STAFF    OF   THE 

FOREST   PRODUCTS    LABORATORY,    AT  MADISON,  WISCONSIN 

IN    APPRECIATION    OF  THE   MANY   OPPORTUNITIES 

AFFORDED    AND    COURTESIES   EXTENDED 

THE   AUTHOR 


PREFACE 


THIS  book  was  written  primarily  for  students  of  forestry  to 
whom  a  knowledge  of  the  technical  properties  of  wood  is  essential. 
The  mechanics  involved  is  reduced  to  the  simplest  terms  and 
without  reference  to  higher  mathematics,  with  which  the  students 
rarely  are  familiar.  The  intention  throughout  has  been  to  avoid 
all  unnecessarily  technical  language  and  descriptions,  thereby 
making  the  subject-matter  readily  available  to  every  one  interested 
in  wood. 

Part  I  is  devoted  to  a  discussion  of  the  mechanical  properties 
of  wood — the  relation  of  wood  material  to  stresses  and  strains. 
Much  of  the  subject-matter  is  merely  elementary  mechanics  of 
materials  in  general,  though  written  with  reference  to  wood  in 
particular.  Numerous  tables  are  included,  showing  the  various 
strength  values  of  many  of  the  more  important  American  woods. 

Part  II  deals  with  the  factors  affecting  the  mechanical  prop- 
erties of  wood.  This  is  a  subject  of  interest  to  all  who  are  con- 
cerned in  the  rational  use  of  wood,  and  to  the  forester  it  also,  by 
retrospection,  suggests  ways  and  means  of  regulating  his  forest 
product  through  control  of  the  conditions  of  production.  Attempt 
has  been  made,  in  the  light  of  all  data  at  hand,  to  answer  many 
moot  questions,  such  as  the  effect  on  the  quality  of  wood  of  rate  of 
growth,  season  of  cutting,  heartwood  and  sapwood,  locality  of 
growth,  weight,  water,  content,  steaming,  and  defects. 

Part  III  describes  methods  of  timber  testing.  They  are  for 
the  most  part  those  followed  by  the  U.  S.  Forest  Service.  In 
schools  equipped  with  the  necessary  machinery  the  instructions 
will  serve  to  direct  the  tests;  in  others  a  study  of  the  text  with 
reference  to  the  illustrations  should  give  an  adequate  conception 
of  the  methods  employed  in  this  most  important  line  of  research. 

The  appendix  contains  a  copy  of  the  working  plan  followed  by 
the  U.  S.  Forest  Service  in  the  extensive  investigations  covering 
the  mechanical  properties  of  the  woods  grown  in  the  United 


vi  PREFACE 

States.  It  contains  many  valuable  suggestions  for  the  inde- 
pendent investigator.  In  addition  four  tables  of  strength  values 
for  structural  timbers,  both  green  and  air-seasoned,  are  included. 
The  relation  of  the  stresses  developed  in  different  structural  forms 
to  those  developed  in  the  small  clear  specimens  is  given. 

In  the  bibliography  attempt  was  made  to  list  all  of  the  im- 
portant publications  and  articles  on  the  mechanical  properties 
of  wood,  and  timber  testing.  While  admittedly  incomplete,  it 
should  prove  of  assistance  to  the  student  who  desires  a  fuller 
knowledge  of  the  subject  than  is  presented  here. 

The  writer  is  indebted  to  the  U.  S.  Forest  Service  for  nearly 
all  of  his  tables  and  photographs  as  well  as  many  of  the  data 
upon  which  the  book  is  based,  since  only  the  Government  is 
able  to  conduct  the  extensive  investigations  essential  to  a  thorough 
understanding  of  the  subject.  More  than  eighty  thousand  tests 
have  been  made  at  the  Madison  laboratory  alone,  and  the  work 
is  far  from  completion. 

The  writer  also  acknowledges  his  indebtedness  to  Mr.  Emanuel 
Fritz,  M.E.,  M.F.,  for  many  helpful  suggestions  in  the  preparation 
of  Part  I;  and  especially  to  Mr.  Harry  Donald, Tiemann,  M.E., 
M.F.,  engineer  in  charge  of  Timber  Physics  at  the  Government 
Forest  Products  Laboratory,  Madison,  Wisconsin,  for  careful 
revision  of  the  entire  manuscript. 

SAMUEL  J.  RECORD. 

YALE  FOREST  SCHOOL,  July  1,  1914. 


CONTENTS 


PAGE 

PREFACE  v 


PART   I 

THE  MECHANICAL  PROPERTIES  OF  WOOD 

Introduction           1 

Fundamental  considerations  and  definitions 2 

Tensile  strength 7 

Compressive  or  crushing  strength 9 

Shearing  strength 19 

Transverse  or  bending  strength:  Beams 22 

Toughness:  Torsion 37 

Hardness 39 

Cleavability 40 

PART   II 

FACTORS  AFFECTING  THE  MECHANICAL  PROPERTIES  OF  WOOD 

Introduction 43 

Rate  of  growth 43 

Heartwood  and  sapwood 50 

Weight,  density,  and  specific  gravity 54 

Color        58 

Cross  grain 59 

Knots 61 

Frost  splits 62 

Shakes,  galls,  pitch  pockets 64 

Insect  injuries 66 

Marine  wood-borer  injuries 67 

Fungous  injuries 68 

Parasitic  plant  injuries 70 

Locality  of  growth 70 

Season  of  cutting 73 

Water  content 75 

Temperature 84 

Preservatives 86 

vii 


Vlll  CONTENTS 

PART  III 

TIMBER  TESTING  PAGE 

Working  plan    . 88 

Forms  of  material  tested 88 

Size  of  test  specimens 89 

Moisture  determination 90 

Machine  for  static  tests , 90 

Speed  of  testing  machine 92 

Bending  large  beams 94 

Bending  small  beams 99 

Endwise  compression 102 

Compression  across  the  grain        104 

Shear  along  the  grain 107 

Impact  test        110 

Hardness  test:  Abrasion  and  indentation 114 

Cleavage  test 118 

Tension  test  parallel  to  the  grain 118 

Tension  test  at  right  angles  to  the  grain 120 

Torsion  test 122 

Special  tests      . 123 

Spike  pulling  test        123 

Packing  boxes        124 

Vehicle  and  implement  woods 124 

Cross-arms 124 

Other  tests 125 

APPENDIX 

Sample  working  plan  of  United  States  Forest  Service 127 

Strength  values  for  structural  timbers        138 

BIBLIOGRAPHY 145 

Part  I:  Some  general  works  on  mechanics,  materials  of  construction, 

and  testing  of  materials 147 

Part  II:  Publications  and  articles  on  the  mechanical  properties  of 

wood,  and  timber  testing  148 

Part  III:  Publications  of  the  United  States  Government  on  the 

mechanical  properties  of  wood,  and  timber  testing    .     .     .     .157 

INDEX    ,                                                                                                             .  161 


ILLUSTRATIONS 


FIG.  PAGE 

Photomicrograph  of  a  small  block  of  western  hemlock  Frontispiece 

1. — Stress-strain  diagrams  of  two  longleaf  pine  beams 4 

2. — Compression  across  the  grain 10 

3. — Side  view  of  failures  in  compression  across  the  grain 10 

4. — End  view  of  failures  in  compression  across  the  grain 10 

5. — Testing  a  buggy -spoke  in  endwise  compression 11 

6. — Unequal  distribution    of    stress    in    a    long  column  due  to  lateral 

bending 12 

7. — Endwise  compression  of  a  short  column 15 

8. — Failures  of  a  short  column  of  green  spruce 17 

9. — Failures  of  short  columns  of  dry  chestnut 18 

10.— Example  of  shear  along  the  grain 19 

11. — Failures  of  test  specimens  in  shear  along  the  grain 19 

12. — Horizontal  shear  in  a  beam 21 

13. — Oblique  shear  in  a  short  column 21 

14. — Failure  of  a  short  column  by  oblique  shear 21 

15. — Diagram  of  a  simple  beam 23 

16. — Three   common   forms   of   beams — (1)    simple,    (2)    cantilever,    (3) 

continuous 24 

17. — Characteristic  failures  of  simple  beams 35 

18. — Failure  of  a  large  beam  by  horizontal  shear 37 

19. — Torsion  of  a  shaft 38 

20. — Effect  of  torsion  on  different  grades  of  hickory 39 

21. — Cleavage  of  highly  elastic  wood 41 

22. — Cross-sections  of  white  ash,  red  gum,  and  eastern  hemlock      ...  45 

23. — Cross-section  of  longleaf  pine 46 

24. — Relation  of  the  moisture  content  to  the  various  strength  values  of 

spruce 77 

25. — Cross-section  of  the  wood  of   western  larch  showing  fissures  in  the 

thick-walled  cells  of  the  late  wood 79 

26. — Progress  of  drying  throughout  the  length  of  a  chestnut  beam     .     .  79 

27. — Excessive  season  checking 81 

28. — Control  of  season  checking  by  the  use  of  $-irons 83 

29. — Static  bending  test  on  a  large  beam 95 

30. — Two  methods  of  loading  a  beam 97 

31. — Static  bending  test  on  a  email  beam 99 

32, -Sample  log  sheet,  giving  full  details  of  a  transverse  bending  test  on 

a  small  pine  beam 101 

ix 


X  ILLUSTRATIONS 

FIG.  PAGE 

33. — Endwise  compression  test  103 

34. — Sample  log  sheet  of  an  endwise  compression  test  on  a  short  pine 

column 105 

35. — Compression  across  the  grain 106 

36. — Vertical  section  of  shearing  tool 107 

37. — Front  view  of  shearing  tool 108 

38. — Two  forms  of  shear  test  specimens  108 

39. — Making  a  shearing  test 109 

40. — Impact  testing  machine Ill 

41. — Drum  record  of  impact  bending  test 113 

42. — Abrasion  machine  for  testing  the  wearing  qualities  of  woods  .  .115 
43. — Design  of  tool  for  testing  the  hardness  of  woods  by  indentation  .  .116 

44. — Design  of  tool  for  cleavage  test 117 

45. — Design  of  cleavage  test  specimen 118 

46. — Designs  of  tension  test  specimens  used  in  United  States  .  .  .  .119 
47. — Design  of  tension  test  specimen  used  in  New  South  Wales  .  .  .120 
48. — Design  of  tool  and  specimen  for  testing  tension  at  right  angles  to  the 

grain  121 

49. — Making  a  torsion  test  on  hickory 122 

50. — Method  of  cutting  and  marking  test  specimens 129 

51. — Diagram  of  specific  gravity  apparatus 137 


TABLES 


NO.  PAGE 

I. — Comparative  strength  of  iron,  steel,  and  wood 7 

II. — Ratio  of  strength  of  wood  in  tension  and  in  compression       .     .       8 
III. — Right-angled  tensile  strength  of  small  clear  pieces  of  25  woods 

in  green  condition 9 

IV. — Results  of  compression  tests  across  the  grain  on  51  woods  in 

green  condition,  and  comparison  with  white  oak       .     .     .     .     13 
V. — Relation  of  fibre  stress  at  elastic  limit  in  bending  to  the  crushing 

strength  of  blocks  cut  therefrom  in  pounds  per  square  inch     .     14 
VI. — Results  of  endwise  compression  tests  on  small  clear  pieces  of  40 

woods  in  green  condition        16 

VII. — Shearing  strength  along  the  grain  of  small  clear  pieces  of  41 

woods  in  green  condition        20 

VIII. — Shearing  strength  across  the  grain  of  various  American  woods  .     22 
IX. — Results  of  static  bending  tests  on  small  clear  beams  of  49  woods 

in  green  condition 27 

X. — Results  of  impact  bending  tests  on  small  clear  beams  of  34 

woods  in  green  condition        32 

XI. — Manner  of  first  failure  of  large  beams 36 

XII. — Hardness  of  32  woods  in  green  condition,  as  indicated  by  the 
load  required  to  imbed  a  0.444-inch  steel  ball  to  one-half 

its  diameter 40 

XIII. — Cleavage  strength  of  small  clear  pieces  of  32  woods  in  green 

condition         42 

XIV. — Specific  gravity,  and  shrinkage  of  51  American  woods       ...     56 
XV. — Effect  of  drying  on  the  mechanical  properties  of  wood,  shown  in 
ratio  of  increase  due  to  reducing  moisture  content  from  the 

green  condition  to  kiln-dry 76 

XVI. — Effect  of  steaming  on  the  strength  of  green  loblolly  pine        .     .     85 
XVII. — Speed-strength   moduli,    and  relative  increase  in  strength   at 

rates  of  fibre  strain  increasing  in  geometric  ratio       ....     93 
XVIII. — Results  of  bending  tests  on  green  structural  timbers     ....  140 
XIX. — Results  of  compression   and  shear  tests  on  green  structural 

timbers 141 

XX. — Results  of  bending  tests  on  air-seasoned  structural  timbers    .     .  142 
XXI. — Results  of  compression  and  shear  tests  on  air-seasoned  struc- 
tural timbers 143 

XXII. — Working  unit  stresses  for  structural  timber  expressed  in  pounds 

per  square  inch 144 


PART   I 
THE  MECHANICAL  PROPERTIES . 

INTRODUCTION 

THE  mechanical  properties  of  wood  are  its  fitness  and  ability 
to  resist  applied  or  external  forces.  By  external  force  is  meant 
any  force  outside  of  a  given  piece  of  material  which  tends  to 
deform  it  in  any  manner.  It  is  largely  such  properties  that 
determine  the  use  of  wood  for  structural  and  building  purposes 
and  innumerable  other  uses  of  which  furniture,  vehicles,  im- 
plements, and  tool  handles  are  a  few  common  examples. 

Knowledge  of  these  properties  is  obtained  through  experi- 
mentation either  in  the  employment  of  the  wood  in  practice  or 
by  means  of  special  testing  apparatus  in  the  laboratory.  Owing 
to  the  wide  range  of  variation  in  wood  it  is  necessary  that  a 
great  number  of  tests  be  made  and  that  so  far  as  possible  all 
disturbing  factors  be  eliminated.  For  comparison  of  different 
kinds  or  sizes  a  standard  method  of  testing  is  necessary  and  the 
values  must  be  expressed  in  some  defined  units.  For  these 
reasons  laboratory  experiments  if  properly  conducted  have  many 
advantages  over  any  other  method. 

One  object  of  such  investigation  is  to  find  unit  values  for 
strength  and  stiffness,  etc.  These,  because  of  the  complex 
structure  of  wood,  cannot  have  a  constant  value  which  will  be 
exactly  repeated  in  each  test,  even  though  no  error  be  made. 
The  most  that  can  be  accomplished  is  to  find  average  values,  the 
amount  of  variation  above  and  below,  and  the  laws  which  govern 
the  variation.  On  account  of  the  great  variability  in  strength  of 
different  specimens  of  wood  even  from  the  same  stick  and  appear- 
ing to  be  alike,  it  is  important  to  eliminate  as  far  as  possible  all 
extraneous  factors  liable  to  influence  the  results  of  the  tests. 

The  mechanical  properties  of  wood  considered  in  this  book 
are:  (1)  stiffness  and  elasticity,  (2)  tensile  strength,  (3)  com- 

1 


2         THE  MECHANICAL  PROPERTIES  OF  WOOD 

pressive  or  crushing  strength,  (4)  shearing  strength,  (5)  trans- 
verse or  bending  strength,  (6)  toughness,  (7)  hardness,  (8)  cleav- 
ability,  (9)  resilience.  In  connection  with  these,  associated 
properties  of  importance  are  briefly  treated. 

Ija  making  i>s,«3;  of  .figures  indicating  the  strength  or  other 
mecnaiiical  properties  of  wood  for  the  purpose  of  comparing  the 
relative  merits'  ^cf  different,  species,  the  fact  should  be  borne  in 
mind  that  there  is  a  considerable  range  in  variability  of  each 
individual  material  and  that  small  differences,  such  as  a  few 
hundred  pounds  in  values  of  10,000  pounds,  cannot  be  con- 
sidered as  a  criterion  of  the  quality  of  the  timber.  In  testing 
material  of  the  same  kind  and  grade,  differences  of  25  per  cent 
between  individual  specimens  may  be  expected  in  conifers  and 
50  per  cent  or  even  more  in  hardwoods.  The  figures  given  in  the 
tables  should  be  taken  as  indications  rather  than  fixed  values,  and 
as  applicable  to  a  large  number  collectively  and  not  to  individual 
pieces. 

FUNDAMENTAL  CONSIDERATIONS  AND  DEFINITIONS 

Study  of  the  mechanical  properties  of  a  material  is  concerned 
mostly  with  its  behavior  in  relation  to  stresses  and  strains,  and 
the  factors  affecting  this  behavior.  A  stress  is  a  distributed 
force  and  may  be  defined  as  the  mutual  action  (1)  of  one  body 
upon  another,  or  (2)  of  one  part  of  a  body  upon  another  part. 
In  the  first  case  the  stress  is  external;  in  the  other  internal.  The 
same  stress  may  be  internal  from  one  point  of  view  and  external 
from  another.  An  external  force  is  always  balanced  by  the 
internal  stresses  when  the  body  is  in  equilibrium. 

If  no  external  forces  act  upon  a  body  its  particles  assume 
certain  relative  positions,  and  it  has  what  is  called  its  natural 
shape  and  size.  If  sufficient  external  force  is  applied  the  natural 
shape  and  size  will  be  changed.  This  distortion  or  deformation 
of  the  material  is  known  as  the  strain.  Every  stress  produces 
a  corresponding  strain,  and  within  a  certain  limit  (see  elastic 
limit,  page  5)  the  strain  is  directly  proportional  to  the  stress 
producing  it.*  The  same  intensity  of  stress,  however,  does  not 

*  This  is  in  accordance  with  the  discovery  made  in  1678  by  Robert  Hooke, 
and  is  known  as  Hooke's  law. 


THE  MECHANICAL  PROPERTIES  OF  WOOD         3 

produce  the  same  strain  in  different  materials  or  in  different 
qualities  of  the  same  material.  No  strain  would  be  produced  in 
a  perfectly  rigid  body,  but  such  is  not  known  to  exist. 

Stress  is  measured  in  pounds  (or  other  unit  of  weight  or  force). 
A  unit  stress  is  the  stress  on  a  unit  of  the  sectional  area.  (Unit 

p\ 
stress  =  T  )     For  instance,  if  a  load  (P)  of  one  hundred  pounds 

is  uniformly  supported  by  a  vertical  post  with  a  cross-sectional 
area  (A)  of  ten  square  inches,  the  unit  compressive  stress  is  ten 
pounds  per  square  inch. 

Strain  is  measured  in  inches  (or  other  linear  unit) .  A  unit 
strain  is  the  strain  per  unit  of  length.  Thus  if  a  post  10  inches 
long  before  compression  is  9.9  inches  long  under  the  compressive 
stress,  the  total  strain  .is  0.1  inch,  and  the  unit  strain  is 

j-  =  -~  =  0.01  inch  per  inch  of  length. 
Li         1U 

As  the  stress  increases  there  is  a  corresponding  increase  in  the 
strain.  This  ratio  may  be  graphically  shown  by  means  of  a 
diagram  or  curve  plotted  with  the  increments  of  load  or  stress  as 
ordinates  and  the  increments  of  strain  as  abscissae.  This  is 
known  as  the  stress-strain  diagram.  Within  the  limit  men- 
tioned above  the  diagram  is  a  straight  line.  (See  Fig.  1.)  If 
the  results  of  similar  experiments  on  different  specimens  are 
plotted  to  the  same  scales,  the  diagrams  furnish  a  ready  means 
for  comparison.  The  greater  the  resistance  a  material  offers 
to  deformation  the  steeper  or  nearer  the  vertical  axis  will  be 
the  line. 

There  are  three  kinds  of  internal  stresses,  namely,  (1)  tensile, 
(2)  compressive,  and  (3)  shearing.  When  external  forces  act 
upon  a  bar  in  a  direction  away  from  its  ends  or  a  direct  pull, 
the  stress  is  a  tensile  stress;  when  toward  the  ends  or  a  direct 
push,  compressive  stress.  In  the  first  instance  the  strain  is  an 
elongation;  in  the  second  a  shortening.  Whenever  the  forces 
tend  to  cause  one  portion  of  the  material  to  slide  upon  another 
adjacent  to  it  the  action  is  called  a  shear.  The  action  is  that  of 
an  ordinary  pair  of  shears.  When  riveted  plates  slide  on  each 
other  the  rivets  are  sheared  off. 


4         THE  MECHANICAL  PROPERTIES  OF  WOOD 

These  three  simple  stresses  may  act  together,  producing  com- 
pound stresses,  as  in  flexure.  When  a  bow  is  bent  there  is  a 
compression  of  the  fibres  on  the  inner  or  concave  side  and  an 
elongation  of  the  fibres  on  the  outer  or  convex  side.  There  is 
also  a  tendency  of  the  various  fibres  to  slide  past  one  another  in 
a  longitudinal  direction.  If  the  bow  were  made  of  two  or  more 


15000 


•2  10000 


5000 


.2         A  AB.G          .8         1.0        1.2        1.4        1.6        1.8        2.0        2.2        2A 


FIG.  1. — Stress-strain  diagrams  of  two  longleaf  pine  beams.  E.L.  =  elastic  limit. 
The  areas  of  the  triangles  0(EL)A  and  0(EL)B  represent  the  elastic  resilience  of  the 
dry  and  green  beams,  respectively. 

separate  pieces  of  equal  length  it  would  be  noted  on  bending  that 
slipping  occurred  along  the  surfaces  of  contact,  and  that  the  ends 
would  no  longer  be  even.  If  these  pieces  were  securely  glued 
together  they  would  no  longer  slip,  but  the  tendency  to  do  so  would 
exist  just  the  same.  Moreover,  it  would  be  found  in  the  latter 
case  that  the  bow  would  be  much  harder  to  bend  than  where  the 
pieces  were  not  glued  together — in  other  words,  the  stiffness  of 
the  bow  would  be  materially  increased. 


THE  MECHANICAL  PROPERTIES  OF  WOOD          5 

Stiffness  is  the  property  by  means  of  which  a  body  acted 
upon  by  external  forces  tends  to  retain  its  natural  size  and 
shape,  or  resists  deformation.  Thus  a  material  that  is  difficult 
to  bend  or  otherwise  deform  is  stiff;  one  that  is  easily  bent 
or  otherwise  deformed  is  flexible.  Flexibility  is  not  the  exact 
counterpart  of  stiffness,  as  it  also  involves  toughness  and 
pliability. 

If  successively  larger  loads  are  applied  to  a  body  and  then 
removed  it  will  be  found  that  at  first  the  body  completely  regains 
its  original  form  upon  release  from  the  stress — in  other  words, 
the  body  is  elastic.  No  substance  known  is  perfectly  elastic, 
though  many  are  practically  so  under  small  loads.  Eventually  a 
point  will  be  reached  where  the  recovery  of  the  specimen  is  incom- 
plete. This  point  is  known  as  the  elastic  limit,  which  may  be 
defined  as  the  limit  beyond  which  it  is  impossible  to  carry  the 
distortion  of  a  body  without  producing  a  permanent  alteration 
in  shape.  After  this  limit  has  been  exceeded,  the  size  and  shape 
of  the  specimen  after  removal  of  the  load  will  not  be  the  same  as 
before,  and  the  difference  or  amount  of  change  is  known  as  the 
permanent  set. 

Elastic  limit  as  measured  in  tests  and  used  in  design  may  be 
defined  as  that  unit  stress  at  which  the  deformation  begins  to 
increase  in  a  faster  ratio  than  the  applied  load.  In  practice  the 
elastic  limit  of  a  material  under  test  is  determined  from  the  stress- 
strain  diagram.  It  is  that  point  in  the  line  where  the  diagram 
begins  perceptibly  to  curve.*  (See  Fig.  1,  page  4.) 

Resilience  is  the  amount  of  work  done  upon  a  body  in  deforming 
it.  Within  the  elastic  limit  it  is  also  a  measure  of  the  potential 
energy  stored  in  the  material  and  represents  the  amount  of  work 
the  material  would  do  upon  being  released  from  a  state  of  stress. 
This  may  be  graphically  represented  by  a  diagram  in  which  the 
abscissae  represent  the  amount  of  deflection  and  the  ordinates 
the  force  acting.  The  area  included  between  the  stress-strain 
curve  and  the  initial  line  (which  is  zero)  represents  the  work  done. 
(See  Fig.  1,  page  4.)  If  the  unit  of  space  is  in  inches  and  the  unit 

*  If  the  straight  portion  does  not  pass  through  the  origin,  a  parallel  line 
should  be  drawn  through  the  origin,  and  the  load  at  elastic  limit  taken  from 
this  line.  (See  Fig.  32,  page  101.) 


0  THE  MECHANICAL  PROPERTIES  OF  WOOD 

of  force  is  in  pounds  the  result  is  inch-pounds.  If  the  elastic 
limit  is  taken  as  the  apex  of  the  triangle  the  area  of  the  triangle 
will  represent  the  elastic  resilience  of  the  specimen.  This  amount 
of  work  can  be  applied  repeatedly  and  is  perhaps  the  best  measure 
of  the  toughness  of  the  wood  as  a  working  quality,  though  it  is 
not  synonymous  with  toughness. 

Permanent  set  is  due  to  the  plasticity  of  the  material.  A 
perfectly  plastic  substance  would  have  no  elasticity  and  the 
smallest  forces  would  cause  a  set.  Lead  and  moist  clay  are 
nearly  plastic  and  wood  possesses  this  property  to  a  greater  or 
less  extent.  The  plasticity  of  wood  is  increased  by  wetting, 
heating,  and  especially  by  steaming  and  boiling.  Were  it  not 
for  this  property  it  would  be  impossible  to  dry  wood  without 
destroying  completely  its  cohesion,  due  to  the  irregularity  of 
shrinkage. 

A  substance  that  can  undergo  little  change  in  shape  without 
breaking  or  rupturing  is  brittle.  Chalk  and  glass  are  common 
examples  of  brittle  materials.  Sometimes  the  word  brash  is  used 
to  describe  this  condition  in  wood.  A  brittle  wood  breaks  sud- 
denly with  a  clean  instead  of  a  splintery  fracture  and  without 
warning.  Such  woods  are  unfitted  to  resist  shock  or  sudden 
application  of  load. 

The  measure  of  the  stiffness  of  wood  is  termed  the  modulus 
of  elasticity  (or  coefficient  of  elasticity).  It  is  the  ratio  of  stress 
per  unit  of  area  to  the  deformation  per  unit  of  length. 

/_,       unit  stress\  . 

1  E  =  — .  —  — r-  ]     It  is  a  number  indicative  of  stiffness,  not  of 
V          unit  strain/ 

strength,  and  only  applies  to  conditions  within  the  elastic  limit. 
It  is  nearly  the  same  whether  derived  from  compression  tests  or 
from  tension  tests. 

A  large  modulus  indicates  a  stiff  material.  Thus  in  green  wood 
tested  in  static  bending  it  varies  from  643,000  pounds  per  square 
inch  for  arborvitse  to  1,662,000  pounds  for  longleaf  pine,  and 
1,769,000  pounds  for  pignut  hickory.  (See  Table  IX,  page  27.) 
The  values  derived  from  tests  of  small  beams  of  dry  material  are 
much  greater,  approaching  3,000,000  for  some  of  our  woods. 
These  values  are  small  when  compared  with  steel  which  has  a 


THE  MECHANICAL  PROPERTIES  OF  WOOD         7 

modulus  of  elasticity  of  about  30,000,000  pounds  per  square  inch. 
(See  Table  I.) 

TABLE  I 

COMPARATIVE    STRENGTH    OF    IRON,    STEEL,    AND    WOOD 


!    Modulus  of 

Sp.          elasticity 

Tensile 

Crushing 

Modulus 

MATERIAL                    gr., 

in                  strength 

strength 

of 

dry 

bending 

rupture 

• 

Cast    iron,    cold    blast 
(Hodgkinson)  
Bessemer     steel,     high 
grade  (Fairbain)  .... 

7.1 

7.8 

17,270,000 
29,215,000 

16,700 
88,400 

106,000 
225,600 

38,500 

Longleaf     pine,    3.5% 
moisture  (U.  S.)  .  .  .  . 
Red  spruce,  3.  5%  mois- 
ture (U.  S.)  
Pignut   hickory,  3.5% 
moisture  (U.  S.)  .  .  .  . 

.63 
.41 

.86 

2,800,000 
1,800,000 
2,370,000 

13,000 
8,800 
11,130 

21,000 
14,500 
24,000 

NOTE. — Great  variation  may  be  found  in  different  samples  of  metals  as 
well  as  of  wood.  The  examples  given  represent  reasonable  values. 

TENSILE    STRENGTH 

Tension  results  when  a  pulling  force  is  applied  to  opposite 
ends  of  a  body.  This  external  pull  is  communicated  to  the 
interior,  so  that  any  portion  of  the  material  exerts  a  pull  or  tensile 
force  upon  the  remainder,  the  ability  to  do  so  depending  upon  the 
property  of  cohesion.  The  result  is  an  elongation  or  stretching 
of  the  material  in  the  direction  of  the  applied  force.  The  action 
is  the  opposite  of  compression. 

Wood  exhibits  its  greatest  strength  in  tension  parallel  to  the 
grain,  and  it  is  very  uncommon  in  practice  for  a  specimen  to  be 
pulled  in  two  lengthwise.  This  is  due  to  the  difficulty  of  making 
the  end  fastenings  secure  enough  for  the  full  tensile  strength  to  be 
brought  into  play  before  the  fastenings  shear  off  longitudinally. 
This  is  not  the  case  with  metals,  and  as  a  result  they  are  used  in 
almost  all  places  where  tensile  strength  is  particularly  needed, 
even  though  the  remainder  of  the  structure,  such  as  sills,  beams, 
joists,  posts,  and  flooring,  may  be  of  wood.  Thus  in  a  wooden 
truss  bridge  the  tension  members  are  steel  rods. 


8         THE  MECHANICAL  PROPERTIES  OF  WOOD 

The  tensile  strength  of  wood  parallel  to  the  grain  depends 
upon  the  strength  of  the  fibres  and  is  affected  not  only  by  the  nature 
and  dimensions  of  the  wood  elements  but  also  by  their  arrange- 
ment. It  is  greatest  in  straight-grained  specimens  with  thick- 
walled  fibres.  Cross  grain  of  any  kind  materially  reduces  the 
tensile  strength  of  wood,  since  the  tensile  strength  at  right  angles 
to  the  grain  is  only  a  small  fraction  of  that  parallel  to  the  grain. 

TABLE  II 

RATIO    OF    STRENGTH    OF    WOOD    IN    TENSION    AND    IN    COMPRESSION 

(Bul.  10,  U.  S.  Div.  of  Forestry,  p.  44) 


KIND  OF  WOOD 


TJ    ,.  A  stick  1  square  inch  in 

Tensile  CTOSS  section' 

strength       !  Wei^ht  re^uired  to~ 


compressive 


strength  pull  aparfc        I    Crugh  cndwise 


Hickory  

3.7 

32,000 

8,500 

Elm  

3.8 

29,000 

7,500 

Larch  

2.3 

19,400 

8,600 

Longleaf  pine  

2.2 

17,300 

7,400 

NOTE. — Moisture  condition  not  given. 

Failure  of  wood  in  tension  parallel  to  the  grain  occurs  some- 
times in  flexure,  especially  with  dry  material.  The  tension 
portion  of  the  fracture  is  nearly  the  same  as  though  the  piece  were 
pulled  in  two  lengthwise.  The  fibre  walls  are  torn  across  obliquely 
and  usually  in  a  spiral  direction.  There  is  practically  no  pulling 
apart  of  the  fibres,  that  is,  no  separation  of  the  fibres  along  their 
walls,  regardless  of  their  thickness.  The  nature  of  tension  failure 
is  apparently  not  affected  by  the  moisture  condition  of  the 
specimen,  at  least  not  so  much  so  as  the  other  strength  values.* 

Tension  at  right  angles  to  the  grain  is  closely  related  to  cleava- 
bility.  When  wood  fails  in  this  manner  the  thin  fibre  walls  are 
torn  in  two  lengthwise  while  the  thick-walled  fibres  are  usually 
pulled  apart  along  the  primary  wall. 


*  See  Brush,  Warren  D.:     A  microscopic  study  of  the  mechanical  failure 
of  wood.     Vol.  II,  Rev.  F.  S.  Investigations,  Washington,  D.  C.,  1912,  p.  35. 


THE   MECHANICAL   PROPERTIES   OF   WOOD 


TABLE  III 

TENSILE   STRENGTH   AT   RIGHT   ANGLES   TO   THE    GRAIN   OF   SMALL   CLEAR   PIECES 
OF    25    WOODS    IN    GREEN    CONDITION 

(Forest  Service  Cir.  213) 


COMMON  NAME  OF  SPECIES 

When  surface  of  failure 
is  radial 

When  surface  of  failure 
is  tangential 

Hardwoods 
Ash,  white  
Basswood  
Beech  

Lbs.  per  sq.  inch 

645 
226 
633 

Lbs.  per  sq.  inch 

671 
303 
969 

Birch,  yellow  
Elm,  slippery  

446 
765 

526 

832 

Hackberry  

661 

786 

Locust,  honey 

1  133 

1  445 

Maple,  sugar  
Oak,  post  
red  
swamp  white  
white  

610 
714 
639 
757 

622 

864 
924 
874 
909 
749 

yellow  
Sycamore  
Tupelo.  . 

728 
540 
472 

929 
781 
796 

Conifers 
Arborvitse  
Cypress,  bald  
Fir,  white  
Hemlock 

241 
242 
213 
271 

235 
251 
304 
323 

Pine,  longleaf  
red  
sugar 

240 
179 
239 

298 
205 
304 

western  yellow  
white  .  .  
Tamarack  

230 
225 
236 

252 

285 
274 

COMPRESSIVE    OR    CRUSHING    STRENGTH 

Compression  across  the  grain  is  very  closely  related  to  hard- 
ness and  transverse  shear.  There  are  two  ways  in  which  wood 
is  subjected  to  stress  of  this  kind,  namely,  (1)  with  the  load 
acting  over  the  entire  area  of  the  specimen,  and  (2)  with  a  load 
concentrated  over  a  portion  of  the  area.  (See  Fig.  2.)  The 
latter  is  the  condition  more  commonly  met  with  in  practice,  as, 
for  example,  where  a  post  rests  on  a  horizontal  sill,  or  a  rail  rests 
on  a  cross-tie.  The  former  condition,  however,  gives  the  true 
resistance  of  the  grain  to  simple  crushing. 


10 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


Steel 

Plate 

1 

The  first  effect  of  compression  across  the  grain  is  to  compact 
the  fibres,  the  load  gradually  but  irregularly  increasing  as  the 

density  of  the  material 
is  increased.  If  the 
specimen  lies  on  a  flat 
surface  and  the  load  is 
applied  to  only  a  por- 
tion of  the  upper  area, 
the  bearing  plate  in- 
dents the  wood,  crush- 
ing the  upper  fibres 
without  affecting  the 

FIG.  2. — Compression  across  the  grain. 

lower  part,     (feee  .big. 

3.)     As  the  load  increases  the  projecting  ends  sometimes  split 
horizontally.      (See  Fig.  4.)     The  irregularities  in  the  load  are 


b 


d 


FIG.  3. — Side  view  of  failures  in  compression  across  the  grain,  showing  crushing  of 
blocks  under  bearing  plate.     Specimen  at  right  shows  splitting  at  ends. 

due  to   the  fact  that  the   fibres    collapse   a    few    at    a    time, 
beginning  with  those  with  the  thinnest  walls.     The  projection 


FIG.  4. — End  view  of  failures  in  compression  across  the  grain,  showing  splitting  of 
the  ends  of  the  test  specimens. 


Photo  by  U.  S.  Forest  Service 


FIG.  5. — Testing  a  buggy  spoke  in  endwise  compression,  illustrating  the  failure  by 
sidewise  bending  of  a  long  column  fixed  only  at  the  lower  end. 


12 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


of  the  ends  increases  the  strength  of  the  material  directly 
beneath  the  compressing  weight  by  introducing  a  beam  action 
which  helps  support  the  load.  This  influence  is  exerted  for  a 
short  distance  only. 

When  wood  is  used  for  columns,  props,  posts,  and  spokes, 
the  weight  of  the  load  tends  to  shorten  the  ma- 
terial endwise.  This  is  endwise  compression,  p 
or  compression  parallel  to  the  grain.  In  the  case 
of  long  columns,  that  is,  pieces  in  which  the  length 
is  very  great  compared  with  their  diameter,  the 
failure  is  by  sidewise  bending  or  flexure,  instead  of 
by  crushing  or  splitting.  (See  Fig.  5.)  A  familiar 
instance  of  this  action  is  afforded  by  a  flexible  walk- 
ing-stick. If  downward  pressure  is  exerted  with 
the  hand  on  the  upper  end  of  the  stick  placed  verti- 
cally on  the  floor,  it  will  be  noted  that  a  definite 
amount  of  force  must  be  applied  in  each  instance 
before  decided  flexure  takes  place.  After  this 
point  is  reached  a  very  slight  increase  of  pressure 
very  largely  increases  the  deflection,  thus  obtaining 
so  great  a  leverage  about  the  middle  section  as  to 
cause  rupture. 

The  lateral  bending  of  a  column  produces  a 
combination  of  bending  with  compressive  stress 
over  the  section,  the  compressive  stress  being 
maximum  at  the  section  of  greatest  deflection  on 
the  concave  side.  The  convex  surface  is  under 
tension,  as  in  an  ordinary  beam  test.  (See  Fig. 
6.)  If  the  same  stick  is  braced  in  such  a  way 
that  flexure  is  prevented,  its  supporting  strength  is 
increased  enormously,  since  the  coniDressive  stress 
acts  uniformly  over  the  section,  and  failure  is  by 
crushing  or  splitting,  as  in  small  blocks.  In  all 
columns  free  to  bend  in  any  direction  the  deflection 
will  be  seen  in  the  direction  in  which  the  column  is 
least  stiff.  This  sidewise  bending  can  be  overcome 
by  making  pillars  and  columns  thicker  in  the  middle  than  at 
the  ends,  and  by  bracing  studding,  props,  and  compression 


FIG.  G. — 
Unequal  dis- 
tribution of 
stress  in  a  long 
column  due  to 
lateral  bend- 
ing. 


THE    MECHANICAL    PROPERTIES    OF   WOOD  13 

TABLE  IV 

RESULTS   OF   COMPRESSION   TESTS   ACROSS   THE    GRAIN   ON   51    WOODS    IN    GREEN 
CONDITION,    AND    COMPARISON    WITH    WHITE    OAK 

(U.  S.  Forest  Service) 


COMMON  NAME  OF  SPECIES 


Fibre  stress  at  elastic 

limit  perpendicular  to 

grain 


Fibre  stress  in  per 
cent  of  white  oak,  or 
853  pounds  per  sq.  in. 


Lbs.  per  sfj.  inch 

Osage  orange 2,260 

Honey  locust 1,684 

Black  locust 1,426 

Post  oak 1,148 

Pignut  hickory 1,142 

Water  hickory 1,088 

Shagbark  hickory 1,070 

Mockernut  hickory 1,012 

Big  shellbark  hickory 997 

Bitternut  hickory 986 

Nutmeg  hickory 938 

Yellow  oak 857 

White  oak |  853 

Bur  oak 836 

White  ash !  828 

Red  oak 778 

Sugar  maple 742 

Rock  elm 696 

Beech 607 

Slippery  elm 599 

Redwood 578 

Bald  cypress 548 

Red  maple 531 

Hackberry 525 

Incense  cedar    518 

Hemlock 497 

Longleaf  pine I  491 

Tamarack I  480 

Silver  maple 456 

Yellow  birch    I  454 

Tupelo [  451 

Black  cherry 

Sycamore 433 

Douglas  fir 427 

Cucumber  tree 408 

Shortleaf  pine 400 

Red  pine ...    358 

Sugar  pine 353 

White  elm 351 

Western  yellow  pine 348 

Lodgepole  pine 

Red  spruce 345 

White  pine 314 

Engelman  spruce 290 

Arborvitae 288 

Largetooth  aspen 269 

White  spruce 262 

Butternut 258 

Buckeye  (yellow) 210 

Basswood > 209 

Black  willow..               193 


Per  cent 

265.0 

197.5 

167.2 

134.6 

133.9 

127.5 

125.5 

118.6 

116.9 

115.7 

110.0 

100.5 

100.0 

98.0 

97.1 

91.2 

87.0 

81.6 

71.2 

70.2 

67.8 

64.3 

62.3 

61.6 


60.8 
58.3 
57.6 
56.3 
53.5 
53.2 
52.9 
52.1 
50.8 
50.1 
47.8 
46.9 
42.0 
41.4 
41.2 
40.8 
40.8 
40.5 
36.8 
34.0 
33.8 
31.5 
30.7 
30.3 
24.6 
24.5 
22.6 


14 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


members  of  trusses.      The  strength  of  a    column    also    depends 
to   a   considerable  extent  upon   whether   the   ends   are  free  to 

turn  or  are  fixed. 

TABLE  V 

RELATION  OF  FIBRE  STRESS  AT  ELASTIC  LIMIT   (r)  IN  BENDING  TO  THE  CRUSHING 
STRENGTH   (C)   OF  BLOCKS  CUT  THEREFROM,  IN  POUNDS  PER  SQUARE  INCH 

(Forest  Service  Bui.  70,  p.  90) 
LONGLEAF  PINE 


MOISTURE  CONDITION 

Soaked 
50  per 
cent 

Green 
23  per 
cent 

14rer 
cent 

11.5  per 
cent 

9.5  per 
cent 

Kiln-dry 
6.2  per 
cent 

Number  of  tests  averaged. 

5 

5 

5 

5 

4 

5 

r  in  bending  

4,920 

5,944 

6,924 

7,852 

9,280 

11,550 

C  in  compression  

4,668 

5,100 

6,466 

7,466 

8,985 

10,910 

Per  cent  r  is  in  excess  of  C  . 

5.5 

16.5 

7.1 

5.2 

3.3 

5.9 

SPRUCE 


MOISTURE  CONDITION 

Soaked 
30  per 

Green 
30  per 

10  per 

8.1  per 

Kiln-dry 
3.9  per 

cent 

cent 

cent 

Number  of  tests  averaged  5 

4 

5 

3 

4 

r  in  bending  

3,002 

3,362 

6,458 

8,400 

10,170 

C  in  compression  

2,680 

3,025 

6,120 

7,610 

9,335 

Per  cent  r  is  in  excess  of  C  12.0 

11.1 

5.5 

10.4 

9.0 

The  complexity  of  the  computations  depends  upon  the  way  in 
which  the  stress  is  applied  and  the  manner  in  which  the  stick 
bends.  Ordinarily  where  the  length  of  the  test  specimen  is  not 
greater  than  four  diameters  and  the  ends  are  squarely  faced 
(see  Fig.  7),  the  force  acts  uniformly  over  each  square  inch  of  area 
and  the  crushing  strength  is  equal  to  the  maximum  load  (P) 

/          p\ 
divided  by  the  area  of  the  cross-section  (A).     (  C  =  -7  J 

It  has  been  demonstrated*  that  the  ultimate  strength  in  com- 
pression parallel  to  the  grain  is  very  nearly  the  same  as  the  extreme 


*  See  Circular  No.  18,  U.  S.  Division  of  Forestry:  Progress  in  timber 
physics,  pp.  13-18;  also  Bulletin  70,  U.  S.  Forest  Service:  Effect  of  moisture 
on  the  strength  and  stiffness  of  wood,  pp.  42,  89-90. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


15 


fibre  stress  at  the  elastic  limit  in  bending.  (See  Table  V.)  In 
other  words,  the  transverse  strength  of  beams  at  elastic  limit  is 
practically  equal  to  the  compressive  strength  of  the  same  material 
in  short  columns.  It  is  accordingly  possible  to  calculate  the 
approximate  breaking  strength  of  beams  from  the  compressive 
strength  of  short  columns  except  when  the  wood  is  brittle.  Since 
tests  on  endwise  compression  are  simpler,  easier  to  make,  and  less 
expensive  than  transverse  bending  tests, 
the  importance  of  this  relation  is  obvious, 
though  it  does  not  do  away  with  the  neces- 
sity of  making  beam  tests. 

When  a  short  column  is  compressed 
until  it  breaks,  the  manner  of  failure  depends 
partly  upon  the  anatomical  structure  and 
partly  upon  the  degree  of  humidity  of  the 
wood.  The  fibres  (tracheids  in  conifers) 
act  as  hollow  tubes  bound  closely  together, 
and  in  giving  way  they  either  (1)  buckle,  or 
(2)  bend.* 

The  first  is  typical  of  any  dry  thin- 
walled  cells,  as  is  usually  the  case  in  sea- 
soned white  pine  and  spruce,  and  in  the 
early  wood  of  hard  pines,  hemlock,  and 
other  species  with  decided  contrast  between 
the  two  portions  of  the  growth  ring.  As  a 
rule  buckling  of  a  tracheid  begins  at  the  FlG  7 
bordered  pits  which  form  places  of  least 
resistance  in  the  walls.  In  hardwoods  such 

as  oak,  chestnut,  ash,  etc.,  buckling  occurs  only  in  the  thinnest- 
walled  elements,  such  as  the  vessels,  and  not  in  the  true  fibres. 

According  to  Jaccard  f  the  folding  of  the  cells  is  accompanied 
by  characteristic  alterations  of  their  walls  which  seem  to  split 
them  into  extremely  thin  layers.  When  greatly  magnified,  these 
layers  appear  in  longitudinal  sections  as  delicate  threads  without 


Endwise  compres- 
sion of  a  short  column. 


*See  Bulletin  70,  op.  cit.,  p.  129. 

f  Jaccard,  P.:  Etude  anatomique  des  bois  comprimes.  Mit.  d.  Schw. 
Centralanstalt  f.  d.  forst.  Versuchswesen.  X.  Band,  1.  Heft.  Zurich,  1910, 
p.  66. 


16 


THE    MECHANICAL    PROPERTIES    OF   WOOD 


TABLE  VI 

RESULTS    OF    ENDWISE     COMPRESSION    TESTS    ON    SMALL    CLEAR    PIECES    OF    40 
WOODS    IN    GREEN    CONDITION 

(Forest  Service  Cir.  213) 


COMMON  NAME  OF  SPECIES 

Fibre  stress 
at  elastic 
limit 

Crushing 
strength 

Modulus 
of 
elasticity 

Hardwoods 

Ash,  white  
Basswood/  
Beech  
Birch,  yellow  
Elm,  slippery 

Lbs.  per  sq. 
inch 

3,510 
780 
2,770 
2,570 
3  410 

Lbs.  per  sq. 
inch 

4,220 
1,820 
3,480 
3,400 
3,990 

Lbs.  per  sq.  inch 

,531,000 
,016,000 
,412,000 
,915,000 
,453,000 

Hackberry  
Hickory,  big  shellbark 

2,730 
3,570 

3,310 
4,520 

,068,000 
,658,000 

bitternut  
mockernut  
nutmeg  
pignut 

4,330 
3,990 
3,620 
3  520 

4,570 
4,320 
3,980 
4,820 

,616,000 
,359,000 
,411,000 
980  000 

shagbark  

3,730 

4,600 

,943,000 

water  
Locust,  honey  
Maple,  sugar  .    . 

3,240 
4,300 
3,040 

4,660 
4,970 
3,670 

,926,000 
,536,000 
,463,000 

Oak,  post  
red 

2,780 
2  290 

3,330 
3  210 

,062,000 
295  000 

swamp  white  
white  

3,470 
2400 

4,360 
3,520 

,489,000 
946  000 

yellow  

2870 

3,700 

,465  000 

Osage  orange  
Sycamore  .    .    . 
Tupelo  

3,980 
2,320 

2,280 

5,810 
2,790 
3,550 

,331,000 
,073,000 
,280,000 

Conifers 
Arborvitsp.  .  . 

1  420 

1  990 

754  000 

Cedar,  incense.  .  . 

2  710 

3  030 

868  000 

Cypress,  bald  
Fir,  alpine  

3,560 
1  660 

3,960 
2,060 

1,738,000 

882  000 

amabilis  
Douglas  
white  .  .  . 

2,763 
2,390 
2  610 

3,040 
2,920 
2  800 

1,579,000 
1,440,000 
1  332  000 

Hemlock  
Pine,  lodgepole  
longleaf  
red  
sugar  
western  yellow  
white  •  
Redwood  
Spruce,  Engelmann 

2,110 
2,290 
3,420 
2,470 
2,340 
2,100 
2,370 
3,420 
1  880 

2,750 
2,530 
4,280 
3,080 
2,600 
2,420 
2,720 
3,820 
2  170 

1,054,000 
,219,000 
,890,000 
,646,000 
,029,000 
,271,000 
,318,000 
,175,000     -. 
021  000 

Tamarack  

3,010 

3  480 

596  000 

THE  MECHANICAL  PROPERTIES  OF  WOOD        17 

any  definite  arrangements,  while  on  cross  section  they  appear  as 
numerous  concentric  strata.  This  may  be  explained  on  the 
ground  that  the  growth  of  a  fibre  is  by  successive  layers  which, 
under  the  influence  of  compression,  are  sheared  apart.  This  is 
particularly  the  case  with  thick-walled  cells  such  as  are  found 
in  late  wood. 

The  second  case,  where  the  fibres  bend  with  more  or  less 
regular  curves  instead  of  buckling,  is  characteristic  of  any  green 
or  wet  wood,  and  in  dry  woods  where  the  fibres  are  thick-walled. 
In  woods  in  which  the  fibre  walls  show  all  gradations  of  thickness— 
in  other  words,  where  the  transition  from  the  thin-walled  cells  of 


FIG.  8. — Failures  of  short  columns  of  green  spruce. 

the  early  wood  to  the  thick-walled  cells  of  the  late  wood  is  gradual 
—the  two  kinds  of  failure,  namely,  buckling  and  bending,  grade 
into  each  other.  In  woods  with  very  decided  contrast  between 
early  and  late  wood  the  two  forms  are  usually  distinct.  Except 
in  the  case  of  complete  failure  the  cavity  of  the  deformed  cells 
remains  open,  and  in  hardwoods  this  is  true  not  only  of  the  wood 
fibres  but  also  of  the  tube-like  vessels.  In  many  cases  longitudinal 
splits  occur  which  isolate  bundles  of  elements  by  greater  or 
less  intervals.  The  splitting  occurs  by  a  tearing  of  the  fibres 
or  rays  and  not  by  the  separation  of  the  rays  from  the  adjacent 
elements. 

Moisture  in  wood  decreases  the  stiffness  of  the  fibre  walls  and 
enlarges  the  region  of  failure.     The  curve  which  the  fibre  walls 


18        THE  MECHANICAL  PROPERTIES  OF  WOOD 

make  in  the  region  of  failure  is  more  gradual  and  also  more  irregu- 
lar than  in  dry  wood,  and  the  fibres  are  more  likely  to  be  separated. 
In  examining  the  lines  of  rupture  in  compression  parallel  to  the 
grain  it  appears  that  there  does  not  exist  any  specific  type,  that  is, 
one  that  is  characteristic  of  all  woods.  Test  blocks  taken  from 
different  parts  of  the  same  log  may  show  very  decided  differences 


FIG.  9. — Failures  of  short  columns  of  dry  chestnut. 

in  the  manner  of  failure,  while  blocks  that  are  much  alike  in  the 
size,  number,  and  distribution  of  the  elements  of  unequal  re- 
sistance may  behave  very  similarly.  The  direction  of  rupture 
is,  according  to  Jaccard,  not  influenced  by  the  distribution  of  the 
medullary  rays.*  These  are  curved  with  the  bundles  of  fibres 
to  which  they  are  attached.  In  any  case  the  failure  starts  at 


*  This  does  not  correspond  exactly  with  the  conclusions  of  A.  Thil,  who 
says  ("Constitution  anatomique  du  bois,"  pp.  140-141):  "The  sides  of  the 
medullary  rays  sometimes  produce  planes  of  least  resistance  varying  in  size 
with  the  height  of  the  rays.  The  medullary  rays  assume  a  direction  more 
or  less  parallel  to  the  lumen  of  the  cells  on  which  they  border;  the  latter  curve 
to  the  right  or  left  to  make  room  for  the  ray  and  then  close  again  beyond  it. 
If  the  force  acts  parallel  to  the  axis  of  growth,  the  tracheids  are  more  likely 
to  be  displaced  if  tne  marginal  cells  of  the  medullary  rays  are  provided  with 
weak  walls  that  are  readily  compressed.  This  explains  why  on  the  radial 
surface  of  the  test  blocks  the  plane  of  rupture  passes  in  a  direction  nearly 
following  a  medullary  ray,  whereas  on  the  tangential  surface  the  direction 
of  the  plane  of  rupture  is  oblique — but  with  an  obliquity  varying  with  the 
species  and  determined  by  the  pitch  of  the  spirals  along  which  the  medullary 
rays  are  distributed  in  the  stem."  See  Jaccard,  op.  cit.,  pp.  57  et  seq. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


19 


the  weakest  points  and  follows  the  lines  of  least  resistance.  The 
plane  of  failure,  as  visible  on  radial  surfaces,  is  horizontal,  and  on 
the  tangential  surface  it  is  diagonal. 

SHEARING    STRENGTH 

Whenever  forces  act  upon  a  body  in  such  a  way  that  one 
portion  tends  to  slide  upon  another  adjacent  to  it  the  action 
is  called  a  shear.*  In  wood 
this  shearing  action  may  be  (1) 
along  the  grain,  or  (2)  across  the 
grain.  A  tenon  breaking  out 
its  mortise  is  a  familiar  example 
of  shear  along  the  grain,  while 
the  shoving  off  of  the  tenon  itself 
would  be  shear  across  the  grain. 
The  use  of  wood  for  pins  or  tree- 
nails involves  resistance  to  shear 

acrOSS  the  grain.      Another  COm-    FIG.  10.— Example  of  shear  along  the  grain. 

mon  instance  of  the   latter  is  where  the  steel  edge  of  the  eye  of 

an  axe  or  hammer 
tends  to  cut  off  the 
handle.  In  Fig. 
10  the  action  of 
the  wooden  strut 
tends  to  shear  off 
along  the  grain  the 
portion  AB  of  the 
wooden  tie  rod, 
and  it  is  essential 
that  the  length  of 
this  portion  be 
great  enough  to 
guard  against  it. 
Fig.  11  shows 
characteristic  fail- 

FIG.  11. — Failures  of  test  specimens  in  shear  along  the        Ures  in  shear  along 
grain.     In  the  block  at  the  left  the  surface  of  failure  Is          , 
radial;  in  the  one  at  the  right,  tangenital.  tne  gram- 


*  Shear  should  not  be  confused  with  ordinary  cutting  or  incision. 


20 


THE    MECHANICAL    PROPERTIES    OF   WOOD 
TABLE  VII 


SHEARING  STRENGTH  ALONG  THE   GRAIN  OF  SMALL  CLEAR  PIECES  OF  41   WOODS 
IN    GREEN    CONDITION 

(Forest  Service  Cir.  213) 


COMMON  NAME  OF  SPECIES 

When  surface  of  failure 
is  radial 

When  surface  of  failure 
is  tangential 

Lbs.  per  sq.  inch 

Los.  per  sq.  inch 

Hardwoods 

Ash,  black  

876 

832 

white  

1,360 

1,312 

Basswood  

560 

617 

Beech  

1,154 

1,375 

Birch,  yellow  

1,103 

1,188 

Elm,  slippery 

1,197 

1,174 

white  

778 

872 

Hackberry  

1,095 

1,161 

Hickory,  big  shellbark  

1,134 

1,191 

bitternut  

1,134 

1,348 

mockernut  

1,251 

1,313 

nutmeg  

1,010 

1,053 

pignut  

1,334 

1,457 

shagbark  

1,230 

1,297 

water  

1,390 

1,490 

Locust,  honey  

1,885 

2,096 

Maple,  red  

1,130 

1,330 

sugar  

1,193 

1,455 

Oak,  post  

1,198 

1,402 

red  

1,132 

1,195 

swamp  white  

1,198 

1,394 

white 

1,096 

1,292 

yellow  

1,162 

1,196 

Sycamore  

900 

1,102 

Tupelo  

978 

1,084 

Conifers 

Arborvitai.  

617 

614 

Cedar,  incense  

613 

662 

Cypress,  bald  
Fir,  alpine  

836 
573 

800 
654 

amabilis  

517 

639 

Douglas  

853 

858 

white  

742 

723 

Hemlock  

790 

813 

Pine,  lodgepole  

672 

747 

longleaf  

1,060 

953 

red  

812 

741 

sugar  

702 

714 

western  yellow  

686 

706 

white  

649 

639 

Spruce,  Engelmann  
Tamarack  

607 

883 

624 

843 

THE  MECHANICAL  PROPERTIES  OF  WOOD 


21 


Both  shearing  stresses  may  act  at  the  same  time.  Thus 
the  weight  carried  by  a  beam  tends  to  shear  it  off  at  right  angles 
to  the  axis;  this  stress  is  equal  to  the  resultant  force  acting  per- 
pendicularly at  any  point,  and  in  a  beam  uniformly  loaded  and 


FIG.  12. — Horizontal  shear  in  a  beam. 


supported  at  either  end  is  maximum  at  the  points  of  support  and 
zero  at  the  centre.  In  addition  there  is  a  shearing  force  tending 
to  move  the  fibres  of  the  beam  past  each  other  in  a  longitudinal 
direction.  (See  Fig.  12.)  This  longitudinal  shear  is  maximum 
at  the  neutral  plane  and  decreases  to- 
ward the  upper  and 

lower  surfaces. 

Shearing    across   the 

grain  is  so  closely  related 

to  compression  at  right 

angles  to  the  grain  and 

to    hardness   that   there 

is  little  to  be  gained  by 

making    separate    tests 

upon  it.     Knowledge  of 

shear     parallel     to    the 

grain  is  important,  since 

wood  frequently  fails  in 

that  way.    The  value  of 

shearing    stress    parallel 

to  the  grain  is  found  by 

dividing    the    maximum 
load  in  pounds  (P)  by  the  area  of  the  cross  section  in  inches  (A). 


Shear  = 


Oblique  shearing  stresses  are  developed  in  a  bar  when  it  is 
subjected    to    direct    tension    or    compression.     The    maximum 


F  I  G.  13. — 
Oblique  shear 
in  a  short  col- 
umn. 


10.  14. — Failure   of  short   col- 
umn by  oblique  shear. 


22 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


shearing  stress  occurs  along  a  plane  when  it  makes  an  angle  of 
45  degrees  with  the  axis  of  the  specimen.     In  this  case,  shear  = 
p 
2~j.     When  the  value  of  the  angle  0  is  less  than  45  degrees,  the 

p 

shear  along   the   plane  =  j  sin  0  cos  0.     (See  Fig.  13.)      The 

effect  of  oblique  shear  is  often  visible  in  the  failures  of  short 
columns.     (See  Fig.  14.) 

TABLE  VIII 

SHEARING    STRENGTH    ACROSS    THE    GRAIN    OF    VARIOUS    AMERICAN    WOODS 

(J.  C.  Trautwine.     Jour.  Franklin  Institute.  Vol.  109,  1880,  pp.  105-106) 


KIND  OF  WOOD 

Lbs.  per  sq. 
inch 

KIND  OF  WOOD 

Lbs.  per  sq. 
inch 

Ash 

6  280 

Hickory 

7285 

Beech 

5  223 

Locust 

7  176 

Birch 

5  595 

Maple                   

6,355 

Cedar  (white) 

1  372 

Oak                   

4,425 

Cedar  (white) 

1  519 

Oak  (live)      

8,480 

Cedar  (Central  Amer  ) 

3410 

Pine  (white)  

2,480 

Cherry 

2,945 

Pine  (northern  yellow)  

4,340 

Chestnut 

1,536 

Pine  (southern  yellow)  

5,735 

Dogwood 

6,510 

Pine  (very  resinous  yellow). 

5,053 

Ebony 

7,750 

Poplar  

4,418 

Gum 

5  890 

Spruce 

3  255 

Hemlock 

2750 

Walnut  (black) 

4728 

Hickory 

6045 

Walnut  (common) 

2,830 

NOTE. — Two  specimens  of  each  were  tested, 
without  defects.  The  piece  sheared  off  was  ^ 
of  each  pin  was  0.322  sq.  in. 


All  were  fairly  seasoned  and 
in.     The  single  circular  area 


TRANSVERSE    OR    BENDING    STRENGTH  I    BEAMS 

When  external  forces  acting  in  the  same  plane  are  applied  at 
right  angles  to  the  axis  of  a  bar  so  as  to  cause  it  to  bend,  they 
occasion  a  shortening  of  the  longitudinal  fibres  on  the  concave  side 
and  an  elongation  of  those  on  the  convex  side.  Within  the 
elastic  limit  the  relative  stretching  and  contraction  of  the  fibres  is 
directly  *  proportional  to  their  distances  from  a  plane  inter- 

*  While  in  reality  this  relationship  does  not  exactly  hold,  the  formulae  for 
beams  are  based  on  its  assumption. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


23 


mediate  between  them — the  neutral  plane.  (NiP  in  Fig.  15.) 
Thus  the  fibres  half-way  between  the  neutral  plane  and  the 
outer  surface  experience  only  half  as  much  shortening  or  elonga- 
tion as  the  outermost  or  extreme  fibres.  Similarly  for  other 
distances.  The  elements  along  the  neutral  plane  experience  no 
tension  or  compression  in  an  axial  direction.  The  line  of  inter- 
section of  this  plane  and  the  plane  of  section  is  known  as  the 
neutral  axis  (NA  in  Fig.  15)  of  the  section. 

If  the  bar  is  symmetrical  and  homogeneous  the  neutral  plane 
is  located  half-way  between  the  upper  and  lower  surfaces,  so  long 
as  the  deflection  does  not  exceed  the  elastic  limit  of  the  material. 


FIG.  15.- — Diagram  of  a'  simple  beam.     X1  P  =  neutral  plane,  N~  A  =  neutral  axis  of 

section  R  S. 


Owing  to  the  fact  that  the  tensile  strength  of  wood  is  from  two 
to  nearly  four  times  the  compressive  strength,  it  follows  that  at 
rupture  the  neutral  plane  is  much  nearer  the  convex  than  the 
concave  side  of  the  bar  or  beam,  since  the  sum  of  all  the  com- 
pressive stresses  on  the  concave  portion  must  always  equal  the 
sum  of  the  tensile  stresses  on  the  convex  portion.  The  neutral 
plane  begins  to  change  from  its  central  position  as  soon  as  the 
elastic  limit  has  been  passed.  Its  location  at  any  time  is.  very 
uncertain. 

The  external  forces  acting  to  bend  the  bar  also  tend  to  rupture 
it  at  right  angles  to  the  neutral  plane  by  causing  one  transverse 
section  to  slip  past  another.  This  stress  at  any  point  is  equal 
to  the  resultant  perpendicular  to  the  axis  of  the  forces  acting 
at  this  point,  and  is  termed  the  transverse  shear  (or  in  the  case 
of  beams,  vertical  shear). 


24 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


In  addition  to  this  there  is  a  shearing  stress,  tending  to  move 
the  fibres  past  one  another  in  an  axial  direction,  which  is  called 
longitudinal  shear  (or  in  the  case  of  beams,  horizontal  shear). 
This  stress  must  be  taken  into  consideration  in  the  design  of  timber 
structures.  It  is  maximum  at  the  neutral  plane  and  decreases  to 
zero  at  the  outer  elements  of  the  section.  The  shorter  the  span 
of  a  beam  in  proportion  to  its  height,  the  greater  is  the  liability 
of  failure  in  horizontal  shear  before  the  ultimate  strength  of  the 
beam  is  reached. 

Beams 

There  are  three  common  forms  of  beams,  as  follows : 

(1)  Simple  beam — a  bar  resting  upon  two  supports,  one  near 
each  end.     (See  Fig.  16,  No.  1.) 

(2)  Cantilever  beam  —  a  bar   resting   upon  one   support  or 


-Span- 


J 


FIG.  16. — Three  common  forms  of  beams.      1.  Simple.     2.  Cantilever. 
3.  Continuous. 


fulcrum,  or  that  portion  of  any  beam  projecting  out  of  a  wall  or 
beyond  a  support.     (See  Fig.  16,  No.  2.) 

(3)  Continuous  beam — a  bar  resting  upon  more  than  two 
supports.     (See  Fig.  16,  No.  3.) 


THE  MECHANICAL  PROPERTIES  OF  WOOD        25 

Stiffness  of  Beams 

The  two  main  requirements  of  a  beam  are  stiffness  and  strength. 
The  formula  for  the  modulus  of  elasticity  (E)  or  measure  of 
stiffness  of  a  rectangular  prismatic  simple  beam  loaded  at  the 
centre  and  resting  freely  on  supports  at  either  end  is:* 

F  P'1* 

~  4  D  b  h* 

b    =  breadth  or  width  of  beam,  inches. 

h    =  height  or  depth  of  beam,  inches. 

I     =  span  (length  between  points  of  supports)  of  beam,  inches. 

D  =  deflection  produced  by  load  P',  inches. 

P1 '  =  load  at  or  below  elastic  limit,  pounds. 

From  this  formula  it  is  evident  that  for  rectangular  beams 
of  the  same  material,  mode  of  support,  and  loading,  the  deflection 
is  affected  as  follows: 

(1)  It  is  inversely  proportional  to  the  width  for  beams  of  the 
same  length  and  depth.     If  the  width  is  tripled  the  deflection 
is  one-third  as  great. 

(2)  It  is  inversely  proportional  to  the  cube  of  the  depth  for 
beams  of  the  same  length  and  breadth.     If  the  depth  is  tripled 
the  deflection  is  one  twenty-seventh  as  great. 

(3)  It  is  directly  proportional  to  the  cube  of  the  span  for 
beams  of  the  same  breadth  and  depth.     Tripling  the  span  gives 
twenty-seven  times  the  deflection. 

The  number  of  pounds  which  concentrated  at  the  centre  will 
deflect  a  rectangular  prismatic  simple  beam  one  inch  may  be 
found  from  the  preceding  formula  by  substituting  D  =  I"  and 
solving  for  P' '.  The  formula  then  becomes: 

4  E  b  h3 

Necessary  weight  (P')  =         ,- — 

In  this  case  the  values  for  E  are  read  from  tables  prepared  from 
data  obtained  by  experimentation  on  the  given  material. 


*  Only  this  form  of  beam  is  considered  since  it  is  the  simplest.  For  can- 
tilever and  continuous  beams,  and  beams  rigidly  fixed  at  one  or  both  ends,  as 
well  as  for  different  methods  of  loading,  different  forms  of  cross  section,  etc., 
other  formulae  are  required.  See  any  book  on  mechanics. 


26  THE    MECHANICAL   PROPERTIES    OF   WOOD 

Strength  of  Beams 

The  measure  of  the  breaking  strength  of  a  beam  is  expressed 
in  terms  of  unit  stress  by  a  modulus  of  rupture,  which  is  a  purely 
hypothetical  expression  for  points  beyond  the  elastic  limit.  The 
formula  used  in  computing  this  modulus  is  as  follows: 

1.5  P  I 


R  = 


bh* 


b,  h,  I  =  breadth,  height,  and  span,  respectively,  as  in  pre- 
ceding formula. 

R  =  modulus  of  rupture,  pounds  per  square  inch. 

P  =  maximum  load,  pounds. 

In  calculating  the  fibre  stress  at  the  elastic  limit  the  same 
formula  is  used  except  that  the  load  at  elastic  limit  (Pi)  is  sub- 
stituted for  the  maximum  load  (P). 

From  this  formula  it  is  evident  that  for  rectangular  prismatic 
beams  of  the  same  material,  mode  of  support,  and  loading,  the 
load  which  a  given  beam  can  support  varies  as  follows: 

(1)  It  is  directly  proportional  to  the  breadth  for  beams  of  the 
same  length  and  depth,  as  is  the  case  with  stiffness. 

(2)  It  is  directly  proportional  to  the  square  of  the  height  for 
beams  of  the  same  length  and  breadth,  instead  of  as  the  cube  of 
this  dimension  as  in  stiffness. 

(3)  It  is  inversely  proportional  to  the  span  for  beams  of  the 
same  breadth  and  depth  and  not  to  the  cube  of  this  dimension 
as  in  stiffness. 

The  fact  that  the  strength  varies  as  the  square  of  the  height 
and  the  stiffness  as  the  cube  explains  the  relationship  of  bending 
to  thickness.  Were  the  law  the  same  for  strength  and  stiffness 
a  thin  piece  of  material  such  as  a  sheet  of  paper  could  not  be 
bent  any  further  without  breaking  than  a  thick  piece,  say  an  inch 
board. 

Kinds  of  Loads 

There  are  various  ways  in  which  beams  are  loaded,  of  which 
the  following  are  the  most  important: 

(1)  Uniform  load  occurs  where  the  load  is  spread  evenly  over 
the  beam. 


THE   MECHANICAL   PROPERTIES    OF   WOOD 


27 


TABLE  IX 

RESULTS    OF    STATIC    BENDING    TESTS    ON    SMALL    CLEAR    BEAMS    OF    49    WOODS 
IN    GREEN    CONDITION 

(Forest  Service  Cir.  213) 


COMMON  NAME  OF 

SPECIES 

Fibre 

stress 
at 
clastic 
limit 

Modulus 
of 
rupture 

Modulus 
of 
elasticity 

Work  in  bending 

To 
elastic 
limit 

To 
maximum 
load 

Total 

Hardwoods 
Ash   black  

Lbs.  per 
sq.  in. 

2,580 
5,180 
2,480 
4,490 
4,190 
4,290 
5,560 
2,850 
3,460 
3,320 

6,370 
5,470 
6,550 
4,860 
5,860 
6,120 
5,980 
6,020 
4,450 
4,630 
4,720 
3,490 
5,380 
6,580 
4,320 
5,060 
7,760 
2,820 
4,300 

2,600 
3,950 
4,430 
2,366 
4,060 
3,570 
3,880 
3,410 
3,080 
5,090 
3,740 
4,360 
3,330 
3,180 
3,410 
4,530  ~ 
2,740 
3,440 
3,160 
4,200 

Lbs.  per 
sq.  in. 

6,000 
9,920 
4,450 
8,610 
8,390 
9,430 
9,510 
6,940 
6,450 
7,800 

11,110 
10,280 
11,110 
9,060 
11,810 
11,000 
10,740 
12,360 
8,310 
8,860 
7,380 
7,780 
9,860 
10,710 
8,090 
8,570 
13,660 
6,300 
7,380 

4,250 
6,040 
7,110 
4,450 
6,570 
6,340 
5,970 
5,770 
5,130 
8,630 
6,430 
7,710 
5,270 
5,180 
5,310 
6,560 
4,550 
5,820 
5,200 
7,170 

Lbs.  per  sq. 
in. 

960,000 
1,416,000 
842,000 
1,353,000 
1,597,000 
1,222,000 
,314,000 
,052,000 
,138,000 
,170,000 

,562,000 
,399,000 
,508,000 
,289,000 
,769,000 
,752,000 
,563,000 
1,732,000 
1,445,000 
1,462,000 
913,000 
,268,000 
,593,000 
,678,000 
,137,000 
,219,000 
1,329,000 
964,000 
1,045,000 

643,000 
754,000 
1,378,000 
861,000 
1,323,000 
1,242,000 
1,131,000 
917,000 
1,015,000 
1,662,000 
1,384,000 
1,395,000 
966,000 
1,111,000 
1,073,000 
1,024,000- 
866,000 
1,143,000 
968,000 
1,236,000 

In.-lbs. 
per  cu. 
in. 

0.41 
1.10 
.45 
.96 
.62 
.90 
1.32 
.44 

"!56 

.47 
.22 
.50 
.06 
.12 
.22 
.29 
1.28 
.78 
.88 
1.39 
.60 
1.05 
1.49 
.95 
1.20 
2.53 
.51 
1.00 

.60 

In.-lbs. 
per  cu. 
in. 

13.1 
20.0 
5.8 
14.1 
14.2 
19.4 
11.7 
11.8 

19^6 

24.3 

20.0 
31.7 
22.8 
30.6 
18.3 
18,8 
17.3 
9.8 
12.7 
9.1 
11.4 
14.5 

i2'.l 
11.7 

37.9 
7.1 

7.8 

5.7 

In.-lbs. 
per  cu. 
in. 

38.9 
43.7 
8.9 
31.4 
31.5 
47.4 
44.2 
27.4 

52.9 

78.0 
75.5 
84.4 
58.2 
86.7 
72.3 
52.9 
64.4 
17.1 
32.0 
17  A 
26.0 
37.6 

36.7 
30.7 
101.7 
13.6 
20.9 

9.5 

white    ..... 

Basswood    . 

Beech 

Birch,  yellow  
Elm,  rock  
slippery  
white  . 

Gum    red 

Hackberry  

Hickory, 
big  shellbark  
bitternut  
mockernut  
nutmeg  
pignut      .  . 

shagbark 

water 

Locust   honey 

Maple,  red  
sugar  
Oak,  post  
red  
swamp  white  .  . 
tanbark  
white  
yellow    . 

Osage  orange  
Sycamore  
Tupelo  

Conifers 
Arborvitse  

Cedar,  incense  
Cypress,  bald  
Fir,  alpine  

.96 
.66 

'  !59 
.77 
.73 
.54 
.88 
.59 

"66 

.52 
.62 

".ho 

.62 

.58 
.84 

5.1 
4.4 

'e'.e 

5.2 
6.6 
5.1 

8.1 

5.8 

'S.O 
4.3 
5.9 

'4.8 
6.0 
6.6 
7.2 

15.4 
7.4 

13.Q 

14.9 
12.9 
7.4 
34.8 
28.0 

ii.6 

15.6 
13.3 

'  f  '  i 
s6!6 

amabilis.  .    . 

Douglas.  . 

white 

Hemlock 

Pine,  lodgepole  
longleaf  
red  

shortleaf  
sugar  
west,  yellow.  .  . 
-White  
Redwood  
Spruce,  Engelmann. 
red  
white 

Tamarack  

28  THE    MECHANICAL    PROPERTIES    OF    WOOD 

(2)  Concentrated  load  occurs  where  the  load  is  applied  at 
single  point  or  points. 

(3)  Live   or  immediate  load  is  one  of  momentary  or  short 
duration  at  any  one  point,  such  as  occurs  in  crossing  a  bridge. 

(4)  Dead  or  permanent  load  is  one  of  constant  and  indetermi- 
nate duration,  as  books  on  a  shelf.     In  the   case   of   a   bridge 
the  weight  of  the  structure  itself  is  the  dead  load.     All  large 
beams   support   a  uniform   dead   load    consisting   of   their   own 
weight. 

The  effect  of  dead  load  on  a  wooden  beam  may  be  two  or  more 
times  that  produced  by  an  immediate  load  of  the  same  weight. 
Loads  greater  than  the  elastic  limit  are  unsafe  and  will  generally 
result  in  rupture  if  continued  long  enough.  A  beam  may  be 
considered  safe  under  permanent  load  when  the  deflections 
diminish  during  equal  successive  periods  of  time.  A  continual 
increase  in  deflection  indicates  an  unsafe  load  which  is  almost 
certain  to  rupture  the  beam  eventually. 

Variations  in  the  humidity  of  the  surrounding  air  influence 
the  deflection  of  dry  wood  under  dead  load,  and  increased  deflec- 
tions during  damp  weather  are  cumulative  and  not  recovered  by 
subsequent  drying.  In  the  case  of  longleaf  pine,  dry  beams 
may  with  safety  be  loaded  permanently  to  within  three-fourths 
of  their  elastic  limit  as  determined  from  ordinary  static  tests. 
Increased  moisture  content,  due  to  greater  humidity  of  the  air, 
lowers  the  elastic  limit  of  wood  so  that  what  was  a  safe  load  for 
the  dry  material  may  become  unsafe. 

When  a  dead  load  not  great  enough  to  rupture  a  beam  has 
been  removed,  the  beam  tends  gradually  to  recover  its  former 
shape,  but  the  recovery  is  not  always  complete.  If  specimens 
from  such  a  beam  are  tested  in  the  ordinary  testing  machine  it 
will  be  found  that  the  application  of  the  dead  load  did  not  affect 
the  stiffness,  ultimate  strength,  or  elastic  limit  of  the  material. 
In  other  words,  the  deflections  and  recoveries  produced  by  live 
loads  are  the  same  as  would  have  .been  produced  had  not  the 
beam  previously  been  subjected  to  a  dead  load.* 

*  See  Tiemann,  Harry  D. :  Some  results  of  dead  load  bending  tests  of 
timber  by  means  of  a  recording  deflectometer.  Proc.  Am.  Soc.  for  Testing 
Materials.  Phila.  Vol.  IX,  1909,  pp.  534-548. 


THE  MECHANICAL  PROPERTIES  OF  WOOD         29 

Maximum  load  is  the  greatest  load  a  material  will  support 
and  is  usually  greater  than  the  load  at  rupture. 

Safe  load  is  the  load  considered  safe  for  a  material  to  support 
in  actual  practice.  It  is  always  less  than  the  load  at  elastic  limit 
and  is  usually  taken  as  a  certain  proportion  of  the  ultimate  or 
breaking  load. 

The  ratio  of  the  breaking  to  the  safe  load  is  called  the  factor 

-      -  ,         /-n             ,      ,  ,          ultimate  strength  \ 
of  safety.      (Factor  of  safety  =        7- — j — -, —   )       In  order  to 

make  due  allowance  for  the  natural  variations  and  imperfections 
in  wood  and  in  the  aggregate  structure,  as  well  as  for  variations 
in  the  load,  the  factor  of  safety  is  usually  as  high  as  6  or  10,  es- 
pecially if  the  safety  of  human  life  depends  upon  the  structure. 
This  means  that  only  from  one-sixth  to  one-tenth  of  the  com- 
puted strength  values  is  considered  safe  to  use.  If  the  depth 
of  timbers  exceeds  four  times  their  thickness  there  is  a  great 
tendency  for  the  material  to  twist  when  loaded.  It  is  to  over- 
come this  tendency  that  floor  joists  are  braced  at  frequent  intervals. 
Short  deep  pieces  shear  out  or  split  before  their  strength  in  bending 
can  fully  come  into  play. 

Application  of  Loads 

There  are  three*  general  methods  in  which  loads  may  be 
applied  to  beams,  namely: 

(1)  Static  loading  or  the  gradual  imposition  of  load  so  that 
the  moving  parts  acquire  no  appreciable  momentum.     Loads  are 
so  applied  in  the  ordinary  testing  machine. 

(2)  Sudden  imposition  of  load  without  initial  velocity.    "  Thus 
in  the  case  of  placing  a  load  on  a  beam,  if  the  load  be  brought  into 
contact  with  the  beam,  but  its  weight  sustained  by  external  means, 
as  by  a  cord,  and  then  this  external  support  be  suddenly  (instan- 
taneously) removed,  as  by  quickly  cutting  the  cord,  then,  although 
the  load  is  already  touching  the  beam  (and  hence  there  is  no  real 
impact),  yet  the  beam  is  at  first  offering  no  resistance,  as  it  has 
yet  suffered  no  deformation.    Furthermore,  as  the  beam  deflects  the 


*  A  fourth  might   be  added,  namely,  vibratory,  or  harmonic  repetition, 
which  is  frequently  serious  in  the  case  of  bridges. 


30        THE  MECHANICAL  PROPERTIES  OF  WOOD 

resistance  increases,  but  does  not  come  to  be  equal  to  the  load 
until  it  has  attained  its  normal  deflection.  In  the  meantime 
there  has  been  an  unbalanced  force  of  gravity  acting,  of  a  con- 
stantly diminishing  amount,  equal  at  first  to  the  entire  load,  at  the 
normal  deflection.  But  at  this  instant  the  load  and  the  beam  are 
in  motion,  the  hitherto  unbalanced  force  having  produced  an 
accelerated  velocity,  and  this  velocity  of  the  weight  and  beam 
gives  to  them  an  energy,  or  vis  viva,  which  must  now  spend  itself 
in  overcoming  an  excess  of  resistance  over  and  above  the  imposed 
load,  and  the  whole  mass  will  not  stop  until  the  deflection  (as 
well  as  the  resistance)  has  come  to  be  equal  to  twice  that  corre- 
sponding to  the  static  load  imposed.  Hence  we  say  the  effect  of 
a  suddenly  imposed  load  is  to  produce  twice  the  deflection  and  stress 
of  the  same  load  statically  applied.  It  must  be  evident,  however, 
that  this  case  has  nothing  in  common  with  either  the  ordinary 
'  static  '  tests  of  structural  materials  in  testing-machines,  or  with 
impact  tests."  * 

(3)  Impact,  shock,  or  blow,  f  There  are  various  common  uses 
of  wood  where  the  material  is  subjected  to  sudden  shocks  and  jars 
or  impact.  Such  is  the  action  on  the  felloes  and  spokes  of  a  wagon 
wheel  passing  over  a  rough  road ;  on  a  hammer  handle  when  a  blow 
is  struck;  on  a  maul  when  it  strikes  a  wedge. 

Resistance  to  impact  is  resistance  to  energy  which  is  measured 
by  the  product  of  the  force  into  the  space  through  which  it  moves, 
or  by  the  product  of  one-half  the  moving  mass  which  causes  the 
shock  into  the  square  of  its  velocity.  The  work  done  upon  the 
piece  at  the  instant  the  velocity  is  entirely  removed  from  the 
striking  body  is  equal  to  the  total  energy  of  that  body.  It  is 
impossible,  however,  to  get  all  of  the  energy  of  the  striking  body 
stored  in  the  specimen,  though  the  greater  the  mass  and  the 
shorter  the  space  through  which  it  moves,  or,  in  other  words,  the 
greater  the  proportion  of  weight  and  the  smaller  the  proportion 
of  velocity  making  up  the  energy  of  the  striking  body,  the  more 
energy  the  specimen  will  absorb.  The  rest  is  lost  in  friction,  vibra- 
tions, heat,  and  motion  of  the  anvil. 

*  Johnson,  J.  B.:     The  materials  of  construction,  pp.  81-82. 
f  See  Tiemann,  Harry  D. :    The  theory  of  impact  and  its  application  to 
testing  materials.    Jour.  Franklin  Inst.,  Oct.,  Nov.,  1909,  pp.  235-259, 336-364. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        31 

In  impact  the  stresses  produced  become  very  complex  and 
difficult  to  measure,  especially  if  the  velocity  is  high,  or  the  mass 
of  the  beam  itself  is  large  compared  to  that  of  the  weight. 

The  difficulties  attending  the  measurement  of  the  stresses 
beyond  the  elastic  limit  are  so  great  that  commonly  they  are  not 
reckoned.  Within  the  elastic  limit  the  formula?  for  calculating 
the  stresses  are  based  on  the  assumption  that  the  deflection  is 
proportional  to  the  stress  in  this  case  as  in  static  tests. 

A  common  method  of  making  tests  upon  the  resistance  of 
wood  to  shock  is  to  support  a  small  beam  at  the  ends  and  drop  a 
heavy  weight  upon  it  in  the  middle.  (See  Fig.  40,  page  111.)  The 
height  of  the  weight  is  increased  after  each  drop  and  records  of 
the  deflection  taken  until  failure.  The  total  work  done  upon 
the  specimen  is  equal  to  the  area  of  the  stress-strain  diagram  plus 
the  effect  of  local  inertia  of  the  molecules  at  point  of  contact. 

The  stresses  involved  in  impact  are  complicated  by  the  fact 
that  there  are  various  ways  in  which  the  energy  of  the  striking 
body  may  be  spent: 

(a)  It  produces  a  local  deformation  of  both  bodies  at  the  sur- 
face of  contact,  within  or  beyond  the  elastic  limit.  In  testing 
wood  the  compression  of  the  substance  of  the  steel  striking-weight 
may  be  neglected,  since  the  steel  is  very  hard  in  comparison  with 
the  wood.  In  addition  to  the  compression  of  the  fibres  at  the  sur- 
face of  contact  resistance  is  also  offered  by  the  inertia  of  the  par- 
ticles there,  the  combined  effect  of  which  is  a  stress  at  the  surface 
of  contact  often  entirely  out  of  proportion  to  the  compression 
which  would  result  from  the  action  of  a  static  force  of  the  same 
magnitude.  It  frequently  exceeds  the  crushing  strength  at  the 
extreme  surface  of  contact,  as  in  the  case  of  the  swaging  action  of 
a  hammer  on  the  head  of  an  iron  spike,  or  of  a  locomotive  wheel 
on  the  steel  rail.  This  is  also  the  case  when  a  bullet  is  shot  through 
a  board  or  a  pane  of  glass  without  breaking  it  as  a  whole. 

(6)  It  may  move  the  struck  body  as  a  whole  with1  an  acceler- 
ated velocity,  the  resistance  consisting  of  the  inertia  of  the  body. 
This  effect  is  seen  when  a  croquet  ball  is  struck  with  a  mallet. 

(c)  It  may  deform  a  fixed  body  against  its  external  supports 
and  resistances.  In  making  impact  tests  in  the  laboratory  the 
test  specimen  is  in  reality  in  the  nature  of  a  cushion  between  two 


32 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


impacting  bodies,  namely,  the  striking  weight  and  the  base  of  the 
machine.  It  is  important  that  the  mass  of  this  base  be  sufficiently 
great  that  its  relative  velocity  to  that  of  the  common  centre  of 
gravity  of  itself  and  the  striking  weight  may  be  disregarded. 

(d)  It  may  deform  the  struck  body  as  a  whole  against  the 
resisting  stresses  developed  by  its  own  inertia,  as,  for  example, 
when  a  baseball  bat  is  broken  by  striking  the  ball. 

TABLE  X 

RESULTS   OF  IMPACT  BENDING  TESTS   ON  SMALL   CLEAR  BEAMS   OF  34  WOODS  IN 

GREEN    CONDITION 

(Forest  Service  Cir.  213) 


Fibre  stress 

Work  in  bend- 

COMMON NAME  OF  SPECIES 

at  elastic 

elasticity 

ing  to  elastic 

limit 

limit 

Lbs.  per  sq.  in. 

Lbs.  per  sq.  in. 

In.-lbs.  per  cu. 

Hardwoods 

in. 

Ash,  black  

7,840 

955,000 

3.69 

white  

11,710 

1,564,000 

4.93 

Basswood  

5,480 

917,000 

1.84 

Beech  

11,760 

1,501,000 

5.10 

Birch,  yellow  

11,080 

1,812,000 

3.79 

Elm,  rock  

12,090 

1,367,000 

6.52 

slippery  

11,700 

1,569,000 

4.86 

white  

9,910 

1,138,000 

4.82 

Hackberry  

10,420 

1,398,000 

4.48 

Locust,  honey  

13,460 

2,114,000 

4.76 

Maple,  red  

11,670 

1,411,000 

5.45 

sugar  

11,680 

1,680,000 

4.55 

Oak,  post  

11,260 

,596,000 

4.41 

red  

10,580 

,506,000 

4.16 

swamp  white  

13,280 

2,048,000 

4.79 

white  

9,860 

,414,000 

3.84 

yellow  

10,840 

,479,000 

4.44 

Osage  orange  

15,520 

,498,000 

8.92 

Sycamore  

8,180 

,165,000 

3.22 

Tupelo  

7,650 

1,310,000 

2.49 

Conifers 

\rborvitse 

5,290 

778,000 

2.04 

Cypress,  bald  

8,290 

1,431,000 

2.71 

Fir,  alpine  

5,280 

982,000 

1.59 

Douglas  

8,870 

1,579,000 

2.79 

white  

7,230 

,326,000 

2.21 

Hemlock  

6,330 

,025,000 

2.19 

Pine,  lodgepole  

6,870 

,142,000 

2.31 

longleaf  

9,680 

,739,000 

3.02 

red  

7,480 

,438,000 

2.18 

sugar  

6,740 

,083,000 

2.34 

wrestern  yellow  

7,070 

,115,000 

2.51 

white  

6,490 

,156,000 

2.06 

Spruce,  Engelmann  

6,300 

,076,000 

2.09 

Tamarack  

7,750 

,263,000 

2.67 

THE  MECHANICAL  PROPERTIES  OF  WOOD        33 

Impact  testing  is  difficult  to  conduct  satisfactorily  and  the 
data  obtained  are  of  chief  value  in  a  relative  sense,  that  is,  for 
comparing  the  shock-resisting  ability  of  woods  of  which  like 
specimens  have  been  subjected  to  exactly  identical  treatment. 
Yet  this  test  is  one  of  the  most  important  made  on  wood,  as  it 
brings  out  properties  not  evident  from  other  tests.  Defects  and 
brittleness  are  revealed  by  impact  better  than  by  any  other  kind 
of  test.  In  common  practice  nearly  all  external  stresses  are  of 
the  nature  of  impact.  In  fact,  no  two  moving  bodies  can  come 
together  without  impact  stress.  Impact  is  therefore  the  com- 
monest form  of  applied  stress,  although  the  most  difficult  to 
measure. 

Failures  in  Timber  Beams 

If  a  beam  is  loaded  too  heavily  it  will  break  or  fail  in  some 
characteristic  manner.  These  failures  may  be  classified  according 
to  the  way  in  which  they  develop,  as  tension,  compression,  and 
horizontal  shear;  and  according  to  the  appearance  of  the  broken 
surface,  as  brash,  and  fibrous.  A  number  of  forms  may  develop 
if  the  beam  is  completely  ruptured. 

Since  the  tensile  strength  of  wood  is  on  the  average  about 
three  times  as  great  as  the  compressive  strength,  a  beam  should, 
therefore,  be  expected  to  fail  by  the  formation  in  the  first  place  of  a 
fold  on  the  compression  side  due  to  the  crushing  action,  followed 
by  failure  on  the  tension  side.  This  is  usually  the  case  in  green 
or  moist  wood.  In  dry  material  the  first  visible  failure  is  not 
infrequently  on  the  lower  or  tension  side,  and  various  attempts 
have  been  made  to  explain  why  such  is  the  case.* 

Within  the  elastic  limit  the  elongations  and  shortenings  are 
equal,  and  the  neutral  plane  lies  in  the  middle  of  the  beam.  (See 
page  23.)  Later  the  top  layer  of  fibres  on  the  upper  or  com- 
.pression  side  fail,  and  on  the  load  increasing,  the  next  layer  of 
fibres  fail,  and  so  on,  even  though  this  failure  may  not  be  visible. 
As  a  result  the  shortenings  on  the  upper  side  of  the  beam  become 
considerably  greater  than  the  elongations  on  the  lower  side.  The 
neutral  plane  must  be  presumed  to  sink  gradually  toward  the 
tension  side,  and  when  the  stresses  on  the  outer  fibres  at  the  bottom 

*See  Proc.  Int.  Assn.  for  Testing  Materials,  1912,  XXIIL,  pp.  12-13. 


34  THE   MECHANICAL   PROPERTIES   OF   WOOD 

have  become  sufficiently  great,  the  fibres  are  pulled  in  two,  the 
tension  area  being  much  smaller  than  the  compression  area.  The 
rupture  is  often  irregular,  as  in  direct  tension  tests.  Failure  may 
occur  partially  in  single  bundles  of  fibres  some  time  before  the 
final  failure  takes  place.  One  reason  why  the  failure  of  a  dry 
beam  is  different  from  one  that  is  moist,  is  that  drying  increases 
the  stiffness  of  the  fibres  so  that  they  offer  more  resistance  to 
crushing,  while  it  has  much  less  effect  upon  the  tensile  strength. 

There  is  considerable  variation  in  tension  failures  depending 
upon  the  toughness  or  the  brittleness  of  the  wood,  the  arrangement 
of  the  grain,  defects,  etc.,  making  further  classification  desirable. 
The  four  most  common  forms  are : 

(1)  Simple  tension,  in  which  there  is  a  direct  pulling  in  two  of 
the  wood  on  the  under  side  of  the  beam  due  to  a  tensile  stress 
parallel  to  the  grain.     (See  Fig.  17,  No.  1.)     This  is  common  in 
straight-grained  beams,  particularly  when  the  wood  is  seasoned. 

(2)  Cross-grained  tension,  in  which  the  fracture  is  caused  by  a 
tensile  force  acting  oblique  to  the  grain.     (See  Fig.  17,  No.  2.) 
This  is  a  common  form  of  failure  where  the  beam  has  diagonal, 
spiral  or  other  form  of  cross  grain  on  its  lower  side.     Since  the 
tensile  strength  of  wood  across  the  grain  is  only  a  small  fraction 
of  that  with  the  grain  it  is  easy  to  see  why  a  cross-grained  timber 
would  fail  in  this  manner. 

(3)  Splintering  tension,  in  which  the  failure  consists  of  a  con- 
siderable number  of  slight  tension  failures,  producing  a  ragged  or 
splintery  break  on  the  under  surface  of  the  beam.     (See  Fig.  17, 
No.  3.)     This  is  common  in  tough  woods.     In  this  case  the  surface 
of  fracture  is  fibrous. 

(4)  Brittle  tension,  in  which  the  beam  fails  by  a  clean  break 
extending  entirely  through  it.     (See  Fig.  17,  No.  4.)     It  is  char- 
acteristic of  a  brittle  wood  which  gives  way  suddenly  without 
warning,  like  a  piece  of  chalk.     In  this  case  the  surface  of  fracture 
is  described  as  brash. 

Compression  failure  (see  Fig.  17,  No.  5)  has  few  variations 
except  that  it  appears  at  various  distances  from  the  neutral 
plane  of  the  beam.  It  is  very  common  in  green  timbers.  The 
compressive  stress  parallel  to  the  fibres  causes  them  to  buckle 
or  bend  as  in  an  endwise  compressive  test.  This  action  usually 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


35 


begins  on  the  top  side  shortly  after  the  elastic  limit  is  reached 
and  extends  downward,  sometimes  almost  reaching  the  neutral 
plane  before  complete  failure  occurs.  Frequently  two  or  more 
failures  develop  at  about  the  same  time. 

Horizontal  shear  failure,  in  which  the  upper  and  lower  portions 
of  the  beam  slide  along  each  other  for  a  portion  of  their  length 


FIG.  17. — Characteristic  failures  of  simple  beams. 

either  at  one  or  at  both  ends  (see  Fig.  17,  No.  6),  is  fairly 
common  in  air-dry  material  and  in  green  material  when  the  ratio 
of  the  height  of  the  beam  to  the  span  is  relatively  large.  It  is  not 
common  in  small  clear  specimens.  It  is  often  due  to  shake  or 
season  checks,  common  in  large  timbers,  which  reduce  the  actual 


36 


THE    MECHANICAL    PROPERTIES    OF    WOOD 


TABLE  XI 

MANNER    OF    FIRST    FAILURE    OF    LARGE    BEAMS 

(Forest  Service  Bui.  108,  p.  56) 


COMMON  NAME  OF 

SPECIES 

Total 
number 
of 

tests 

Per  cent  of  total  failing  by 

Tension 

Compression 

Shear 

Longleaf  pine: 
green  
dry 

17 

9 

191 
91 

48 
13 

62 
52 

111 
25 

30 
9 

39 
44 

28 
12 

49 
10 

18 
22 

27 
19 

27 

54 

23 
54 

40 
60 

37 
45 

21 
11 

43 

83 

18 
30 

24 
22 

72 
76 

56 

71 
19 

53 
12 

53 
22 

74 
66 

50 
17 

76 
60 

58 
56 

1 
5 

17 

46 

6 

27 

7 
28 

10 
33 

5 
23 

7 

6 
10 

Douglas  fir: 
green  
dry  
Shortleaf  pine: 
green  
dry  

Western  larch: 
green. 

dry. 

Loblolly  pine: 
green. 

dry  
Tamarack  : 
green  

Western  hemlock: 
green  
dry. 

Redwood  : 
green. 

dry. 

Norway  pine: 
green  
dry  

NOTE. — These    tests  were  made  on  timbers  ranging  in  cross  section  from 
4"  x  10"  to  8"  x  16",  and  with  a  span  of  15  feet. 

area  resisting  the  shearing  action  considerably  below  the  calculated 
area  used  in  the  formula  for  horizontal  shear.  (See  page  98  for  this 
formula.)  For  this  reason  it  is  unsafe,  in  designing  large  timber 
beams,  to  use  shearing  stresses  higher  than  those  calculated  for 
beams  that  failed  in  horizontal  shear.  The  effect  of  a  failure  in 
horizontal  shear  is  to  divide  the  beam  into  two  or  more  beams  the 
combined  strength  of  which  is  much  less  than  that  of  the  original 
beam.  Fig.  18  shows  a  large  beam  in  which  two  failures  in 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


37 


horizontal  shear  occurred  at  the  same  end.  That  the  parts  behave 
independently  is  shown  by  the  compression  failure  below  the 
original  location  of  the  neutral  plane. 

Table  XI  gives  an  analysis  of  the  causes  of  first  failure  in 
840  large  timber  beams  of  nine  different  species  of  conifers.     Of 


Photo  by  U.  S.  Forest  Service. 

FIG.  18. — Failure  of  a  large  beam  by  horizontal  shear. 

the  total  number  tested  165  were  air-seasoned,  the  remainder 
green.  The  failure  occurring  first  signifies  the  point  of  greatest 
weakness  in  the  specimen  under  the  particular  conditions  of  loading 
employed  (in  this  case,  third-point  static  loading). 


TOUGHNESS:  TORSION 

Toughness  is  a  term  applied  to  more  than  one  property  of  wood. 
Thus  wood  that  is  difficult  to  split  is  said  to  be  tough.  Again,  a 
tough  wood  is  one  that  will  not  rupture  until  it  has  deformed 
considerably  under  loads  at  or  near  its  maximum  strength,  or  one 
which  still  hangs  together  after  it  has  been  ruptured  and  may  be 
bent  back  and  forth  without  breaking  apart.  Toughness  includes 
flexibility  and  is  the  reverse  of  brittleness,  in  that  tough  woods 


38        THE  MECHANICAL  PROPERTIES  OF  WOOD 

break  gradually  and  give  warning  of  failure.  Tough  woods  offer 
great  resistance  to  impact  and  will  permit  rougher  treatment  in 
manipulations  attending  manufacture  and  use.  Toughness  is  de- 
pendent upon  the  strength,  cohesion,  quality,  length,  and  arrange- 
ment of  fibre,  and  the  pliability  of  the  wood.  Coniferous  woods 
as  a  rule  are  not  as  tough  as  hardwoods,  of  which  hickory  and 
elm  are  the  best  examples. 

The  torsion  or  twisting  test  is  useful  in  determining  the  tough- 
ness of  wood.  If  the  ends  of  a  shaft  are  turned  in  opposite  direc- 
tions, or  one  end  is  turned  and  the  other  is  fixed,  all  of  the  fibres 
except  those  at  the  axis  tend  to  assume  the  form  of  helices.  (See 
Fig.  19.)  The  strain  produced  by  torsion  or  twisting  is  essentially 


FIG.  19. — Torsion  of  a  shaft. 

shear  transverse  and  parallel  to  the  fibres,  combined  with  longi- 
tudinal tension  and  transverse  compression.  Within  the  elastic 
limit  the  strains  increase  directly  as  the  distance  from  the  axis 
of  the  specimen.  The  outer  elements  are  subjected  to  tensile 
stresses,  and  as  they  become  twisted  tend  to  compress  those  near 
the  axis.  The  elongated  elements  also  contract  laterally.  Cross 
sections  which  were  originally  plane  become  warped.  With 
increasing  strain  the  lateral  adhesion  of  the  outer  fibres  is  de- 
stroyed, allowing  them  to  slide  past  each  other,  and  reducing 
greatly  their  power  of  resistance.  In  this  way  the  strains  on  the 
fibres  nearer  the  axis  are  progressively  increased  until  finally 
all  of  the  elements  are  sheared  apart.  It  is  only  in  the  toughest 
materials  that  the  full  effect  of  this  action  can  be  observed.  (See 
Fig.  20.)  Brittle  woods  snap  off  suddenly  with  only  a  small 
amount  of  torsion,  and  their  fracture  is  irregular  and  oblique 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


39 


to  the  axis  of  the  piece  instead  of  frayed  out  and  more  nearly 
perpendicular  to  the  axis  as  is  the  case  with  tough  woods. 

HARDNESS 

The  term  hardness  is  used  in  two  senses,  namely:  (1)  resistance 
to  indentation,  and  (2)  resistance  to  abrasion  or  scratching.  In  the 
latter  sense  hardness  combined  with  toughness  is  a  measure  of  the 


Black  Ilickorvl          \\\ 


Photo  by  U.  S.  Forest  Service. 

FIG.  20. — Effect  of  torsion  on  different  grades  of  hickory. 

wearing  ability  of  wood  and  is  an  important  consideration  in  the 
use  of  wood  for  floors,  paving  blocks,  bearings,  and  rollers.  While 
resistance  to  indentation  is  dependent  mostly  upon  the  density  of 
the  wood,  the  wearing  qualities  may  be  governed  by  other  factors 
such  as  toughness,  and  the  size,  cohesion,  and  arrangement  of  the 
fibres.  In  use  for  floors,  some  woods  tend  to  compact  and  wear 
smooth,  while  others  become  splintery  and  rough.  This  feature 
is  affected  to  some  extent  by  the  manner  in  which  the  wood  is 
sawed;  thus  edge-grain  pine  flooring  is  much  better  than  flat- 
sawn  for  uniformity  of  wear. 


40 


THE    MECHANICAL    PROPERTIES    OF   WOOD 


TABLE  XII 

HARDNESS    OF   32    WOODS    IN    GREEN    CONDITION,    AS    INDICATED    BY   THE    LOAD 
REQUIRED    TO    IMBED    A    0.444-INCH    STEEL    BALL    TO    ONE-HALF    ITS 
DIAMETER 

(Forest  Service  Cir.  213) 


COMMON   NAME  OP   SPECIES 

Average 

End 
surface 

Radial 
surface 

Tangential 
surface 

Pounds 

Pounds 

Pounds 

Pounds 

Hardwoods 

1  Osage  orange 

1,971 

,838 

2,312 

1,762 

2  Honey  locust 

1,851 

,862 

1,860 

1^832 

3  Swamp  white  oak  .  . 

1,174 

,205 

1,217 

1,099 

4  White  oak  

1,164 

,183 

1,163 

1,147 

5  Post  oak  

1,099 

,139 

1,068 

1,081 

6  Black  oak  

1,069 

,093 

1,083 

1,031 

7  Red  oak 

1,043 

,107 

1,020 

1,002 

8  White  ash  

1,046 

,121 

1,000 

1,017 

9  Beech  

942 

,012 

897 

918 

10  Sugar  maple  

937 

992 

918 

901 

1  1  Rock  elm  

910 

954 

883 

893 

12  Hackberry  

799 

829 

795 

773 

13  Slippery  elm  

788 

919 

757 

687 

14  Yellow  birch  

778 

827 

768 

739 

15  Tupelo  

738 

814 

666 

733 

16  Red  maple  

671 

766 

621 

626 

17  Sycamore.  .  . 

608 

664 

560 

599 

18  Black  ash  

551 

565 

542 

546 

19  White  elm  

496 

536 

456 

497 

20  Basswood  

239 

273 

226 

217 

Conifers 

1  Longleaf  pine  

532 

574 

502 

521 

2  Douglas  fir  

410 

415 

399 

416 

3  Bald  cypress  

390 

460 

355 

354 

4  Hemlock  

384 

463 

354 

334 

5  Tamarack  

384 

401 

380 

370 

6  Red  pine  

347 

355 

345 

340 

7  White  fir. 

346 

381 

322 

334 

8  Western  yellow  pine 

328 

334 

307 

342 

9  Lodgepole  pine  

318 

316 

318 

319 

10  White  pine  

299 

304 

294 

299 

11  Engelmann  spruce..' 

266 

272 

253 

274 

12  Alpine  fir. 

241 

284 

203 

235 

NOTE. — Black  locust  and  hickory  are  not  included  in  this  table,  but  their 
position  would  be  near  the  head  of  the  list. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


41 


Tests  for  either  form  of  hardness  are  of  comparative  value 
only.  Tests  for  indentation  are  commonly  made  by  penetrations 
of  the  material  with  a  steel  punch  or  ball.*  Tests  for  abrasion  are 
made  by  wearing  down  wood  with  sandpaper  or  by  means  of  a 
sand  blast. 

CLEAVABILITY 

Cleavability  is  the  term  used  to  denote  the  facility  with  which 
wood  is  split.  A  splitting  stress  is  one  in  which  the  forces  act 
normally  like  a  wedge.  (See  Fig.  21.)  The  plane  of  cleavage  is 
parallel  to  the  grain,  either  radially  or 
tangentially. 

This  property  of  wood  is  very  impor- 
tant in  certain  uses  such  as  firewood,  fence 
rails,  billets,  and  squares.  Resistance  to 
splitting  or  low  cleavability  is  desirable 
where  wood  must  hold  nails  or  screws,  as 
in  box-making.  Wood  usually  splits  more 
readily  along  the  radius  than  parallel  to 
the  growth  rings  though  exceptions  occur, 
as  in  the  case  of  cross  grain. 

Splitting  involves  transverse  tension, 
but  only  a  portion  of  the  fibres  are  under 
stress  at  a  time.  A  wood  of  little  stiffness 
and  strong  cohesion  across  the  grain  is 
difficult  to  split,  while  one  with  great  stiff- 
ness, such  as  longleaf  pine,  is  easily  split.  The  form  of  the 
grain  and  the  presence  of  knots  greatly  affect  this  quality. 


FIG.  21. — Cleavage  of 
highly  elastic  wood.  The 
cleft  runs  far  ahead  of  the 
wedge. 


See  articles  by  Gabriel  Janka  listed  in  bibliography,  pages  151-152. 


42 


THE    MECHANICAL    PROPERTIES    OF    WOOD 


TABLE  XIII 

CLEAVAGE  STRENGTH  OF  SMALL  CLEAR  PIECES  OF  32  WOODS  IN  GREEN 
CONDITION 

(Forest  Service  Cir.  213) 


COMMON  NAME  OF  SPECIES 

When  surface  of 
failure  is  radial 

When  surface  of 
failure  is  tangential 

Hardwoods 
Ash,  black  

Lbs,  per  sq.  in.  of  width 

275 

Lbs.  per  sq.  in.  of  width 

260 

white 

333 

346 

Basswood 

130 

168 

Beech 

339 

527 

Birch,  yellow  
Elm,  slippery  
white  
Hackberry  
Locust,  honey  
Maple  red 

294 
401 
210 
422 
552 
297 

287 
424 
270 
436 
610 
330 

sugar 

_  376 

513 

Oak,  post  
red  
swamp  white  .  .  .  
white  
yellow  
Sycamore  
Tupelo 

354 
380 

428 
382 
379 
265 
277 

487 
470 
536 
457 
470 
425 
380 

Conifers 
Arborvitae. 

148 

139 

Cypress,  bald  
Fir,  alpine                                        .  . 

167 
130 

154 
133 

Douglas  
white  
Hemlock  
Pine,  lodgepole  
longleaf  .  .  .  
red  
^  sugar 

139 
145 
168 
142 

187 
161 
-168 

127 

187 
151 
140 
180 
154 
189 

western  yellow  
white  

162 
144 

187 
160 

Spruce,  Engelmann  
Tamarack  .  . 

110 
167 

135 

159 

PART  II 

FACTORS   AFFECTING  THE  MECHANICAL 
PROPERTIES  OF  WOOD 

INTRODUCTION 

WOOD  is  an  organic  product — a  structure  of  infinite  variation 
of  detail  and  design.*  It  is  on  this  account  that  no  two  woods  are 
alike — in  reality  no  two  specimens  from  the  same  log  are  identical. 
There  are  certain  properties  that  characterize  each  species,  but 
they  are  subject  to  considerable  variation.  Oak,  for  example,  is 
considered  hard,  heavy,  and  strong,  but  some  pieces,  even  of  the 
same  species  of  oak,  are  much  harder,  heavier,  and  stronger  than 
others.  With  hickory  are  associated  the  properties  of  great 
strength,  toughness,  and  resilience,  but  some  pieces  are  com- 
paratively weak  and  brash  and  ill-suited  for  the  exacting  demands 
for  which  good  hickory  is  peculiarly  adapted. 

It  follows  that  no  definite  value  can  be  assigned  to  the  prop- 
erties of  any  wood  and  that  tables  giving  average  results  of  tests 
may  not  be  directly  applicable  to  any  individual  stick.  With 
sufficient  knowledge  of  the  intrinsic  factors  affecting  the  results 
it  becomes  possible  to  infer  from  the  appearance  of  material  its 
probable  variation  from  the  average.  As  yet  too  little  is  known 
of  the  relation  of  structure  and  chemical  composition  to  the 
mechanical  and  physical  properties  to  permit  more  than  general 
conclusions. 

RATE    OF    GROWTH 

To  understand  the  effect  of  variations  in  the  rate  of  growth  it 
is  first  necessary  to  know  how  wood  is  formed.  A  tree  increases 
in  diameter  by  the  formation,  between  the  old  wood  and  the  inner 
bark,  of  new  woody  layers  which  envelop  the  entire  stem,  living 

*  For  details  regarding  the  structure  of  wood  see  Record,  Samuel  J. : 
Identification  of  the  economic  woods  of  the  United  States.  New  York,  John 
Wiley  &  Sons,  1912. 

43 


44  THE   MECHANICAL   PROPERTIES   OF   WOOD 

branches,  and  roots.  Under  ordinary  conditions  one  layer  is 
formed  each  year  and  in  cross  section  as  on  the  end  of  a  log  they 
appear  as  rings — often  spoken  of  as  annual  rings.  These  growth 
layers  are  made  up  of  wood  cells  of  various  kinds,  but  for  the  most 
part  fibrous.  In  timbers  like  pine,  spruce,  hemlock,  and  other 
coniferous  or  softwood  species  the  wood  cells  are  mostly  of  one 
kind,  and  as  a  result  the  material  is  much  more  uniform  in  struc- 
ture than  that  of  most  hardwoods.  (See  Frontispiece.)  There 
are  no  vessels  or  pores  in  coniferous  wood  such  as  one  sees  so 
prominently  in  oak  and  ash,  for  example.  (See  Fig.  22.) 

The  structure  of  the  hardwoods  is  more  complex.  They  are 
more  or  less  filled  with  vessels,  in  some  cases  (oak,  chestnut,  ash) 
quite  large  and  distinct,  in  others  (buckeye,  poplar,  gum)  too 
small  to  be  seen  plainly  without  a  small  hand  lens.  In  discussing 
such  woods  it  is  customary  to  divide  them  into  two  large  classes 
— ring-porous  and  diffuse-porous.  (See  Fig.  22.)  In  ring-porous 
species,  such  as  oak,  chestnut,  ash,  black  locust,  catalpa,  mulberry, 
hickory,  and  elm,  the  larger  vessels  or  pores  (as  cross  sections  of 
vessels  are  called)  become  localized  in  one  part  of  the  growth 
ring,  thus  forming  a  region  of  more  or  less  open  and  porous  tissue. 
The  rest  of  the  ring  is  made  up  of  smaller  vessels  and  a  much 
greater  proportion  of  wood  fibres.  These  fibres  are  the  elements 
which  give  strength  and  toughness  to  wood,  while  the  vessels  are 
a  source  of  weakness. 

In  diffuse-porous  woods  the  pores  are  scattered  throughout  the 
growth  ring  instead  of  being  collected  in  a  band  or  row.  Examples 
of  this  kind  of  wood  are  gum,  yellow  poplar,  birch,  maple,  cotton- 
wood,  basswood,  buckeye,  and  willow.  Some  species,  such  as 
walnut  and  cherry,  are  on  the  border  between  the  two  classes, 
forming  a  sort  of  intermediate  group. 

If  one  examines  the  smoothly  cut  end  of  a  stick  of  almost  any 
kind  of  wood,  he  will  note  that  each  growth  ring  is  made  up  of 
two  more  or  less  well-defined  parts.  That  originally  nearest  the 
centre  of  the  tree  is  more  open  textured  and  almost  invariably 
lighter  in  color  than  that  near  the  outer  portion  of  the  ring. 
The  inner  portion  was  formed  early  in  the  season,  when  growth 
was  comparatively  rapid  and  is  known  as  early  wood  (also  spring 
wood);  the  outer  portion  is  the  late  wood,  being  produced  in  the 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


45 


summer  or  early  fall.  In  soft  pines  there  is  not  much  contrast  in 
the  different  parts  of  the  ring,  and  as  a  result  the  wood  is  very 
uniform  in  texture  and  is  easy  to  work.  In  hard  pine,  on  the 


Photomicrographs  by  the  author. 

FIG.  22. —  Cross  sections  of  a  ring-porous  hardwood  (white  ash),  a  diffuse-porous 
hardwood  (red  gum),  and  a  non-porous  or  coniferous  wood  (eastern  hemlock).  X  30. 

other  hand,  the  late  wood  is  very  dense  and  is  deep-colored,  pre- 
senting a  very  decided  contrast  to  the  soft,  straw-colored  early 
wood.  (See  Fig.  23.)  In  ring-porous  woods  each  season's  growth 
is  always  well  denned,  because  the  large  pores  of  the  spring  abut 
on  the  denser  tissue  of  the  fall  before.  In  the  diffuse-porous,  the 
demarcation  between  rings  is  not  always  so  clear  and  in  not  a  few 


46 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


cases  is  almost,  if  not  entirely,  invisible  to  the  unaided  eye.    (See 
Fig.  22.) 


Photomicrograph  by  U.  S.  Forest  Service. 

FIG.  23. — Cross  section  of  longleaf  pine  showing  several  growth  rings  with  variations 
in  the  width  of  the  dark -colored  late  wood.     Seven  resin  ducts  are  visible.      X31. 

If  one  compares  a  heavy  piece  of  pine  with  a  light  specimen 
it  will  be  seen  at  once  that  the  heavier  one  contains  a  larger  pro- 


THE  MECHANICAL  PROPERTIES  OF  WOOD        47 

portion  of  late  wood  than  the  other,  and  is  therefore  considerably 
darker.  The  late  wood  of  all  species  is  denser  than  that  formed 
early  in  the  season,  hence  the  greater  the  proportion  of  late  wood 
the  greater  the  density  and  strength.  When  examined  under  a 
microscope  the  cells  of  the  late  wood  are  seen  to  be  very  thick- 
walled  and  with  very  small  cavities,  while  those  formed  first  in 
the  season  have  thin  walls  and  large  cavities.  The  strength  is  in 
the  walls,  not  the  cavities.  In  choosing  a  piece  of  pine  where 
strength  or  stiffness  is  the  important  consideration,  the  principal 
thing  to  observe  is  the  comparative  amounts  of  early  and  late 
wood.  The  width  of  ring,  that  is,  the  number  per  inch,  is  not  nearly 
so  important  as  the  proportion  of  the  late  wood  in  the  ring. 

It  is  not  only  the  proportion  of  late  wood,  but  also  its  quality, 
that  counts.  In  specimens  that  show  a  very  large  proportion  of 
late  wood  it  may  be  noticeably  more  porous  and  weigh  consid- 
erably less  than  the  late  wood  in  pieces  that  contain  but  little. 
One  can  judge  comparative  density,  and  therefore  to  some  extent 
weight  and  strength,  by  visual  inspection. 

The  conclusions  of  the  U.  S.  Forest  Service  regarding  the 
effect  of  rate  of  growth  on  the  properties  of  Douglas  fir  are  sum- 
marized as  follows: 

"  1.  In  general,  rapidly  grown  wood  (less  than  eight  rings  per 
inch)  is  relatively  weak.  A  study  of  the  individual  tests  upon 
which  the  average  points  are  based  shows,  however,  that  when 
it  is  not  associated  with  light  weight  and  a  small  proportion  of 
summer  wood,  rapid  growth  is  not  indicative  of  weak  wood. 

"  2.  An  average  rate  of  growth,  indicated  by  from  12  to  16 
rings  per  inch,  seems  to  produce  the  best  material. 

"  3.  In  rates  of  growths  lower  than  16  rings  per  inch,  the 
average  strength  of  the  material  decreases,  apparently  approaching 
a  uniform  condition  above  24  rings  per  inch.  In  such  slow  rates 
of  growth  the  texture  of  the  wood  is  very  uniform,  and  naturally 
there  is  little  variation  in  weight  or  strength. 

"  An  analysis  of  tests  on  large  beams  was  made  to  ascertain 
if  average  rate  of  growth  has  any  relation  to  the  mechanical  prop- 
erties of  the  beams.  The  analysis  indicated  conclusively  that 
there  was  no  such  relation.  Average  rate  of  growth  [without  con- 
sideration also  of  density],  therefore,  has  little  significance  in 


48        THE  MECHANICAL  PROPERTIES  OF  WOOD 

grading  structural  timber."  *  This  is  because  of  the  wide  vari- 
ation in  the  percentage  of  late  wood  in  different  parts  of  the 
cross  section. 

Experiments  seem  to  indicate  that  for  most  species  there  is  a 
rate  of  growth  which,  in  general,  is  associated  with  the  greatest 
strength,  especially  in  small  specimens.  For  eight  conifers  it  is  as 
follows:  | 

Rings  per  inch 

Douglas  fir 24 

Shortleaf  pine 12 

Loblolly  pine 6 

Western  larch 18 

Western  hemlock 14 

Tamarack 20 

Norway  pine 18 

Redwood 30 

No  satisfactory  explanation  can  as  yet  be  given  for  the  real 
causes  underlying  the  formation  of  early  and  late  wood.  Several 
factors  may  be  involved.  In  conifers,  at  least,  rate  of  growth  alone 
does  not  determine  the  proportion  of  the  two  portions  of  the  ring, 
for  in  some  cases  the  wood  of  slow  growth  is  very  hard  and  heavy, 
while  in  others  the  opposite  is  true.  The  quality  of  the  site  where 
the  tree  grows  undoubtedly  affects  the  character  of  the  wood 
vformed,  though  it  is  not  possible  to  formulate  a  rule  governing 
it.  In  general,  however,  it  may  be  said  that  where  strength  or 
ease  of  working  is  essential,  woods  of  moderate  to  slow  growth 
should  be  chosen.  But  in  choosing  a  particular  specimen  it  is 
not  the  width  of  ring,  but  the  proportion  and  character  of  the 
late  wood  which  should  govern. 

In  the  case  of  the  ring-porous  hardwoods  there  seems  to  exist 
a  pretty  definite  relation  between  the  rate  of  growth  of  timber 
and  its  properties.  This  may  be  briefly  summed  up  in  the  general 
statement  that  the  more  rapid  the  growth  or  the  wider  the  rings 
of  growth,  the  heavier,  harder,  stronger,  and  stiffer  the  wood. 
'This,  it  must  be  remembered,  applies  only  to  ring-porous  woods 
such  as  oak,  ash,  hickory,  and  others  of  the  same  group,  and  is, 
of  course,  subject  to  some  exceptions  and  limitations. 

In  ring-porous  woods  of  good  growth  it  is  usually  the  middle 

*  Bui.  88:  Properties  and  uses  of  Douglas  fir,  p.  29. 

t  Bui.  108,  U.  S.  Forest  Service:  Tests  of  structural  timbers,  p.  37. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        49 

portion  of  the  ring  in  which  the  thick-walled,  strength-giving 
fibres  are  most  abundant.  As  the  breadth  of  ring  diminishes,  this 
middle  portion  is  reduced  so  that  very  slow  growth  produces  com- 
paratively light,  porous  wood  composed  of  thin-walled  vessels  and 
wood  parenchyma.  In  good  oak  these  large  vessels  of  the  early 
wood  occupy  from  6  to  10  per  cent  of  the  volume  of  the  log,  while 
in  inferior  material  they  may  make  up  25  per  cent  or  more.  The 
late  wood  of  good  oak,  except  for  radial  grayish  patches  of  small 
pores,  is  dark  colored  and  firm,  and  consists  of  thick-walled  fibres 
which  form  one-half  or  more  of  the  wood.  In  inferior  oak,  such 
fibre  areas  are  much  reduced  both  in  quantity  and  quality.  Such 
variation  is  very  largely  the  result  of  rate  of  growth. 

Wide-ringed  wood  is  often  called  "  second-growth/'  because 
the  growth  of  the  young  timber  in  open  stands  after  the  old  trees 
have  been  removed  is  more  rapid  than  in  trees  in  the  forest,  and 
in  the  manufacture  of  articles  where  strength  is  an  important 
consideration  such  "  second-growth  "  hardwood  material  is  pre- 
ferred. This  is  particularly  the  case  in  the  choice  of  hickory  for 
handles  and  spokes.  Here  not  only  strength,  but  toughness  and 
resilience  are  important.  The  results  of  a  series  of  tests  on 
hickory  by  the  U.  S.  Forest  Service  show  that  "  the  work  or 
shock-resisting  ability  is  greatest  in  wide-ringed  wood  that  has 
from  5  to  14  rings  per  inch,  is  fairly  constant  from  14  to  38  rings, 
and  decreases  rapidly  from  38  to  47  rings.  The  strength  at  maxi- 
mum load  is  not  so  great  with  the  most  rapid-growing  wood;  it  is 
maximum  with  from  14  to  20  rings  per  inch,  and  again  becomes 
less  as  the  wood  becomes  more  closely  ringed.  The  natural  deduc- 
tion is  that  wood  of  first-class  mechanical  value  shows  from  5  to 
20  rings  per  inch  and  that  slower  growth  yields  poorer  stock. 
Thus  the  inspector  or  buyer  of  hickory  should  discriminate  against 
timber  that  has  more  than  20  rings  per  inch.  Exceptions  exist, 
however,  in  the  case  of  normal  growth  upon  dry  situations,  in 
which  the  slow-growing  material  may  be  strong  and  tough."  * 

The  effect  of  rate  of  growth  on  the  qualities  of  chestnut  wood 
is  summarized  by  the  same  authority  as  follows:  "  When  the 
rings  are  wide,  the  transition  from  spring  wood  to  summer  wood 

*  Bui.  80:  The  commercial  hickories,  pp.  .48-50. 


50        THE  MECHANICAL  PROPERTIES  OF  WOOD 

is  gradual,  while  in  the  narrow  rings  the  spring  wood  passes  into 
summer  wood  abruptly.  The  width  of  the  spring  wood  changes 
but  little  with  the  width  of  the  annual  ring,  so  that  the  narrowing 
or  broadening  of  the  annual  ring  is  always  at  the  expense  of  the 
summer  wood.  The  narrow  vessels  of  the  summer  wood  make  it 
richer  in  wood  substance  than  the  spring  wood  composed  of  wide 
vessels.  Therefore,  rapid-growing  specimens  with  wide  rings  have 
more  wood  substance  than  slow-growing  trees  with  narrow  rings. 
Since  the  more  the  wood  substance  the  greater  the  weight,  and 
the  greater  the  weight  the  stronger  the  wood,  chestnuts  with 
wide  rings  must  have  stronger  wood  than  chestnuts  with  narrow 
rings.  This  agrees  with  the  accepted  view  that  sprouts  (which 
always  have  wide  rings)  yield  better  and  stronger  wood  than 
seedling  chestnuts,  which  grow  more  slowly  in  diameter."  * 

In  diffuse-porous  woods,  as  has  been  stated,  the  vessels  or 
pores  are  scattered  throughout  the  ring  instead  of  collected  in  the 
early  wood.  The  effect  of  rate  of  growth  is,  therefore,  not  the 
same  as  in  the  ring-porous  woods,  approaching  more  nearly  the 
conditions  in  the  conifers.  In  general  it  may  be  stated  that  such 
woods  of  medium  growth  afford  stronger  material  than  when 
very  rapidly  or  very  slowly  grown.  In  many  uses  of  wood,  strength 
is  not  the  main  consideration.  If  ease  of  working  is  prized,  wood 
should  be  chosen  with  regard  to  its  uniformity  of  texture  and 
straightness  of  grain,  which  will  in  most  cases  occur  when  there 
is  little  contrast  between  the  late  wood  of  one  season's  growth  and 
the  early  wood  of  the  next. 

HEARTWOOD    AND    SAPWOOD 

Examination  of  the  end  of  a  log  of  many  species  reveals  a 
darker-colored  inner  portion — the  heartwood,  surrounded  by  a 
lighter-colored  zone — the  sapwood.  In  some  instances  this  dis- 
tinction in  color  is  very  marked;  in  others,  the  contrast  is  slight, 
so  that  it  is  not  always  easy  to  tell  where  one  leaves  off  and  the 
other  begins.  The  color  of  fresh  sapwood  is  always  light,  some- 
times pure  white,  but  more  often  with  a  decided  tinge  of  green 
or  brown. 

*  Bui.  53:  Chestnut  in  southern  Maryland,  pp.  20-21. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        51 

Sapwood  is  comparatively  new  wood.  There  is  a  time  in  the 
early  history  of  every  tree  when  its  wood  is  all  sapwood.  Its 
principal  functions  are  to  conduct  water  from  the  roots  to  the 
leaves  and  to  store  up  and  give  back  according  to  the  season  the 
food  prepared  in  the  leaves.  The  more  leaves  a  tree  bears  and 
the  more  thrifty  its  growth,  the  larger  the  volume  of  sapwood 
required,  hence  trees  making  rapid  growth  in  the  open  have 
thicker  sapwood  for  their  size  than  trees  of  the  same  species 
growing  in  dense  forests.  Sometimes  trees  grown  in  the  open 
may  become  of  considerable  size,  a  foot  or  more  in  diameter,  be- 
fore any  heartwood  begins  to  form,  for  example,  in  second-growth 
hickory,  or  field-grown  white  and  loblolly  pines. 

As  a  tree  increases  in  age  and  diameter  an  inner  portion  of  the 
sapwood  becomes  inactive  and  finally  ceases  to  function.  This 
inert  or  dead  portion  is  called  heartwood,  deriving  its  name  solely 
from  its  position  and  not  from  any  vital  importance  to  the  tree,  as 
is  shown  by  the  fact  that  a  tree  can  thrive  with  its  heart  completely 
decayed.  Some  species  begin  to  form  heartwood  very  early  in 
life,  while  in  others  the  change  comes  slowly.  Thin  sapwood  is 
characteristic  of  such  trees  as  chestnut,  black  locust,  mulberry, 
Osage  orange,  and  sassafras,  while  in  maple,  ash,  gum,  hickory, 
hackberry,  beech,  and  loblolly  pine,  thick  sapwood  is  the  rule. 

There  is  no  definite  relation  between  the  annual  rings  of  growth 
and  the  amount  of  sapwood.  Within  the  same  species  the  cross- 
sectional  area  of  the  sapwood  is  roughly  proportional  to  the  size 
of  the  crown  of  the  tree.  If  the  rings  are  narrow,  more  of  them  are 
required  than  where  they  are  wide.  As  the  tree  gets  larger,  the 
sapwood  must  necessarily  become  thinner  or  increase  materially 
in  volume.  Sapwood  is  thicker  in  the  upper  portion  of  the  trunk 
of  a  tree  than  near  the  base,  because  the  age  and  the  diameter  of 
the  upper  sections  are  less. 

When  a  tree  is  very  young  it  is  covered  with  limbs  almost,  if 
not  entirely,  to  the  ground,  but  as  it  grows  older  some  or  all  of 
them  will  eventually  die  and  be  broken  off.  Subsequent  growth  of 
wood  may  completely  conceal  the  stubs  which,  however,  will 
remain  as  knots.  No  matter  how  smooth  and  clear  a  log  is  on 
the  outside,  it  is  more  or  less  knotty  near  the  middle.  Conse- 
quently the  sapwood  of  an  old  tree,  and  particularly  of  a  forest- 


52        THE  MECHANICAL  PROPERTIES  OF  WOOD 

.grown  tree,  will  be  freer  from  knots  than  the  heartwood.  Since 
in  most  uses  of  wood,  knots  are  defects  that  weaken  the  timber 
and  interfere  with  its  ease  of  working  and  other  properties,  it 
follows  that  sapwood,  because  of  its  position  in  the  tree,  may 
have  certain  advantages  over  heartwood. 

It  is  really  remarkable  that  the  inner  heartwood  of  old  trees 
remains  as  sound  as  it  usually  does,  since  in  many  cases  it  is 
hundreds  of  years,  and  in  a  few  instances  thousands  of  years,  old. 
Every  broken  limb  or  root,  or  deep  wound  from  fire,  insects,  or 
falling  timber,  may  afford  an  entrance  for  decay,  which,  once 
started,  may  penetrate  to  all  parts  of  the  trunk.  The  larvae  of 
many  insects  bore  into  the  trees  and  their  tunnels  remain  indefi- 
nitely as  sources  of  weakness.  Whatever  advantages,  however, 
that  sapwood  may  have  in  this  connection  are  due  solely  to  its 
relative  age  and  position. 

If  a  tree  grows  all  its  life  in  the  open  and  the  conditions  of  soil 
and  site  remain  unchanged,  it  will  make  its  most  rapid  growth  in 
youth,  and  gradually  decline.  The  annual  rings  of  growth  are 
for  many  years  quite  wide,  but  later  they  become  narrower  and 
narrower.  Since  each  succeeding  ring  is  laid  down  on  the  outside 
of  the  wood  previously  formed,  it  follows  that  unless  a  tree  mate- 
rially increases  its  production  of  wood  from  year  to  year,  the  rings 
must  necessarily  become  thinner.  As  a  tree  reaches  maturity  its 
crown  becomes  more  open  and  the  annual  wood  production  is 
lessened,  thereby  reducing  still  more  the  width  of  the  growth 
rings.  In  the  case  of  forest-grown  trees  so  much  depends  upon  the 
competition  of  the  trees  in  their  struggle  for  light  and  nourish- 
ment that  periods  of  rapid  and  slow  growth  may  alternate.  Some 
trees,  such  as  southern  oaks,  maintain  the  same  width  of  ring  for 
hundreds  of  years.  Upon  the  whole,  however,  as  a  tree  gets  larger 
in  diameter  the  width  of  the  growth  rings  decreases. 

It  is  evident  that  there  may  be  decided  differences  in  the  grain 
of  heartwood  and  sapwood  cut  from  a  large  tree,  particularly  one 
that  is  overmature.  The  relationship  between  width  of  growth 
rings  and  the  mechanical  properties  of  wood  is  discussed  under 
Rate  of  Growth.  In  this  connection,  however,  it  may  be  stated 
that  as  a  general  rule  the  wood  laid  on  late  in  the  life  of  a  tree  is 
softer,  lighter,  weaker,  and  more  even-textured  than  that  pro- 


THE  MECHANICAL  PROPERTIES  OF  WOOD        53 

duced  earlier.  It  follows  that  in  a  large  log  the  sapwood,  because 
of  the  time  in  the  life  of  the  tree  when  it  was  grown,  may  be  inferior 
in  hardness,  strength,  and  toughness  to  equally  sound  heartwood 
from  the  same  log. 

After  exhaustive  tests  on  a  number  of  different  woods  the  U.  S. 
Forest  Service  concludes  as  follows:  "  Sapwood,  except  that  from 
old,  overmature  trees,  is  as  strong  as  heartwood,  other  things  being 
equal,  and  so  far  as  the  mechanical  properties  go  should  not  be 
regarded  as  a  defect."  *  Careful  inspection  of  the  individual  tests 
made  in  the  investigation  fails  to  reveal  any  relation  between  the 
proportion  of  sapwood  and  the  breaking  strength  of  timber. 

In  the  study  of  the  hickories  the  conclusion  was:  "  There  is 
an  unfounded  prejudice  against  the  heartwood.  Specifications 
place  white  hickory,  or  sapwood,  in  a  higher  grade  than  red  hick- 
ory, or  heartwood,  though  there  is  no  inherent  difference  in 
strength.  In  fact,  in  the  case  of  large  and  old  hickory  trees,  the 
sapwood  nearest  tire  bark  is  comparatively  weak,  and  the  best 
wood  is  in  the  heart,  though  in  young  trees  of  thrifty  growth  the 
best  wood  is  in  the  sap."  f  The  results  of  tests  from  selected 
pieces  lying  side  by  side  in  the  same  tree,  and  also  the  average 
values  for  heartwood  and  sapwood  in  shipments  of  the  commer- 
cial hickories  without  selection,  show  conclusively  that  "  the  trans- 
formation of  sapwood  into  heartwood  does  not  affect  either  the 
strength  or  toughness  of  the  wood.  ...  It  is  true,  however,  that 
sapwood  is  usually  more  free  from  latent  defects  than  heart- 
wood."  t 

Specifications  for  paving  blocks  often  require  that  longleaf 
pine  be  90  per  cent  heart.  This  is  on  the  belief  that  sapwood  is 
not  only  more  subject  to  decay,  but  is  also  weaker  than  heart- 
wood.  Irreality  there  is  no  sound  basis  for  discrimination  against 
sapwood  on  account  of  strength,  provided  other  conditions  are 
equal.  It  is  true  that  sapwood  will  not  resist  decay  as  long  as 
heartwood,  if  both  are  untreated  with  preservatives.  It  is  espe- 
cially so  of  woods  with  deep-colored  heartwood,  and  is  due  to  infil- 
trations of  tannins,  oils,  and  resins,  which  make  the  wood  more  or 

*Bul.   108:     Tests  of  structural  timber,  p.  35. 
f  Bui.  80:     The  commercial  hickories,  p.  50. 
Loc.  cit. 


54        THE  MECHANICAL  PROPERTIES  OF  WOOD 

less  obnoxious  to  decay-producing  fungi.  If,  however,  the  timbers 
are  to  be  treated,  sapwood  is  not  a  defect;  in  fact,  because  of  the 
relative  ease  with  which  it  can  be  impregnated  with  preservatives 
it  may  be  made  more  desirable  than  heartwood.* 

In  specifications  for  structural  timbers  reference  is  sometimes 
made  to  "  boxheart,"  meaning  the  inclusion  of  the  pith  or  centre 
of  the  tree  within  a  cross  section  of  the  timber.  From  numerous 
experiments  it  appears  that  the  position  of  the  pith  does  not  bear 
any  relation  to  the  strength  of  the  material.  Since  most  season 
checks,  however,  are  radial,  the  position  of  the  pith  may  influence 
the  resistance  of  a  seasoned  beam  to  horizontal  shear,  being  great- 
est when  the  pith  is  located  in  the  middle  half  of  the  section,  t 

WEIGHT,    DENSITY,    AND    SPECIFIC    GRAVITY 

From  data  obtained  from  a  large  number  of  tests  on  the 
strength  of  different  woods  it  appears  that,  other  things  being 
equal,  the  crushing  strength  parallel  to  the  grain,  fibre  stress  at 
elastic  limit  in  bending,  and  shearing  strength  along  the  grain  of 
wood  vary  in  direct  proportion  to  the  weight  of  dry  wood  per  unit 
of  volume  when  green.  Other  strength  values  follow  different 
laws.  The  hardness  varies  in  a  slightly  greater  ratio  than  the 
square  of  the  density.  The  work  to  the  breaking  point  increases 
even  more  rapidly  than  the  cube  of  density.  The  modulus  of 
rupture  in  bending  lies  between  the  first  power  and  the  square  of 
the  density.  This,  of  course,  is  true  only  in  case  the  greater 
weight  is  due  to  increase  in  the  amount  of  wood  substance.  A 

*  Although  the  factor  of  heart  or  sapwood  does  not  influence  the  me- 
chanical properties  of  the  wood  and  there  is  usually  no  difference  in  structure 
observable  under  the  microscope,  nevertheless  sapwood  is  generally  de- 
cidedly different  from  heartwood  in  its  physical  properties.  It  dries  better 
and  more  easily  than  heartwood,  usually  with  less  shrinkage  and  little  checking 
or  honeycombing.  This  is  especially  the  case  with  the  more  refractory  woods, 
such  as  white  oaks  and  Eucalyptus  globulus  and  viminalis.  It  is  usually  much 
more  permeable  to  air,  even  in  green  wood,  notably  so  in  loblolly  pine  and  even 
in  white  oak.  As  already  stated,  it  is  much  more  subject  to  decay.  The 
sapwood  of  white  oak  may  be  impregnated  with  creosote  with  comparative 
ease,  \vhile  the  heartwood  is  practically  impenetrable.  These  facts  indicate 
a  difference  in  its  chemical  nature.— H.  D.  Tiemann. 

t  Bui.  108,  U.  S.  Forest  Service,  p.  36. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        55 

wood  heavy  with  resin  or  other  infiltrated  substance  is  not  neces- 
sarily stronger  than  a  similar  specimen  free  from  such  materials. 
If  differences  in  weight  are  due  to  degree  of  seasoning,  in  other 
words,  to  the  relative  amounts  of  water  contained,  the  rules  given 
above  will  of  course  not  hold,  since  strength  increases  with  dryness. 
But  of  given  specimens  of  pine  or  of  oak,  for  example,  in  the  green 
condition,  the  comparative  strength  may  be  inferred  from  the 
weight.  It  is  not  permissible,  however,  to  compare  such  widely 
different  woods  as  oak  and  pine  on  a  basis  of  their  weights.* 

The  weight  of  wood  substance,  that  is,  the  material  which  com- 
poses the  walls  of  the  fibres  and  other  cells,  is  practically  the  same 
in  all  species,  whether  pine,  hickory,  or  cottonwood,  being  a  little 
greater  than  half  again  as  heavy  as  water.  It  varies  slightly 
from  beech  sapwood,  1.50,  to  Douglas  fir  heartwood,  1.57,  averaging 
about  1.55  at  30°  to  35°  C.,  in  terms  of  water  at  its  greatest  density 
4°  C.  The  reason  any  wood  floats  is  that  the  air  imprisoned  in  its 
cavities  buoys  it  up.  When  this  is  displaced  by  water  the  wood 
becomes  water-logged  and  sinks.  Leaving  out  of  consideration 
infiltrated  substances,  the  reason  a  cubic  foot  of  one  kind  of  dry 
wood  is  heavier  than  that  of  another  is  because  it  contains  a  greater 
amount  of  wood  substance. 

Density  is  merely  the  weight  of  a  unit  of  volume,  as  35  pounds 
per  cubic  foot,  or  0.56  grams  per  cubic  centimetre.  Specific  gravity 
or  relative  density  is  the  ratio  of  the  density  of  any  material  to 
the  density  of  distilled  water  at  4°  C.  (39.2°  F.).  A  cubic  foot  of 
distilled  water  at  4°  C.  weighs  62.43  pounds.  Hence  the  specific 

35 
gravity  of  a  piece  of  wood  with  a  density  of  35  pounds  is  TT^TO 

=  0.561.  To  find  the  weight  per  cubic  foot  when  the  specific  grav- 
ity is  given,  simply  multiply  by  62.43.  Thus,  0.561  X  62.43  =  35. 
In  the  metric  system,  since  the  weight  of  a  cubic  centimetre  of 
pure  water  is  one  gram,  the  density  in  grams  per  cubic  centi- 
metre has  the  same  numerical  value  as  the  specific  gravity. 

Since  the  amount  of  water  in  wood  is  extremely  variable  it 
usually  is  not  satisfactory  to  refer  to  the  density  of  green  wood. 

*  The  oaks  for  some  unknown  reason  fall  below  the  normal  strength  for 
weight,  whereas  the  hickories  rise  above.  Certain  other  woods  also  are  some- 
what exceptional  to  the  normal  relation  of  strength  and  density. 


56 


THE   MECHANICAL   PROPERTIES   OF   WOOD 


TABLE  XIV 

SPECIFIC    GRAVITY,    AND   SHRINKAGE    OF   51    AMERICAN   WOODS 

(Forest  Service  Cir.  213) 


COMMON  NAME 

OF   SPECIES 

Mois- 
ture 
con- 
tent 

Specific  gravity  oven- 
dry,  based  on 

Shrinkage  from  green  to  oven- 
dry  condition 

Volume 
when 
green 

Volume 
when 
oven-dry 

In 
volume 

Radial 

Tangential 

Hardwoods 
Ash,  black. 

Per  cent 
77 
38 
47 
110 
61 
72 
46 
57 
66 
71 
50 

64 
55 
65 
64 
57 
48 
76 
59 
54 
55 
52 
65 
58 
64 
60 
74 
53 
69 
57 
56 
64 
80 
74 
88 
58 
62 
78 
77 
80 
31 
81 
121 

0.466 
.550 
.516 
.315 
.556 
.545 
.578 
.541 
.430 
.434 
.504 

.601 
.666 
.624 
.606 
.662 
.666 
.558 
.627 
.667 
.667 
.667 
.608 
.646 
.617 
.653 
.630 
.695 
.512 
.546 
.577 
.590 
.568 
.637 
.585 
.594 
.603 
.600 
.573 
.550 
.761 
.454 
.475 

0.  640 
.590 
.374 
.669 
.661 

Per  cent 

12.6 
11.7 
14.5 
16.5 
17.0 

15.5 

14.0 

17.6 
20.9 

16.5 
18.9 

15  '6 
15.3 
16.9 
21.2 
16.0 
18.4 

15.5 

'8.6 
14.3 

16.0 
13.1 
17.7 

15.8 
14.3 
16.0 
14.2 

'8.9 
13.5 
12.4 

Per  cent 
4.3 

6.2 
4.6 
7.9 

5.1 

4.2 

7.4 
7.9 

6^9 

8.4 

5.6 
6.3 

6.8 
8.5 
6.5 
7.9 

6^5 

4.9 

5.7 
3.7 
5.5 

6.2 
4.9 

4.8 
4.5 

5'.0 
4.4 

Per  cent 
'6.4 

'8.4 
10.5 
9.0 

'9.9 

'8.9 

11.2 
14.2 

10A 

11.4 

'9.8 
9.5 
10.9 
13.8 
10.2 
11.4 

9.7 

9'i 

10.6 

8.3 
10.6 

'8.3 

9.0 
9.2 
9.7 

'7'.3 
7.9 

white  

n 

Basswood  
Beech  
Birch,  yellow  
Elm,  rock  
slippery  .... 
white  
Gum,  red  
Hackberry  
Hickory, 
big  shellbark.  .  . 

tt              u 

bitternut  

mockernut  
« 

it 

nutmeg  
pignut  

.639 
'576 

^759 
.643 

'732 
.660 
.792 

"!704 
.696 
.708 
.669 

''838 
.526 
.545 

a 

it 

shagbark  

(i 
it 

water  
Locust,  honey.  .  .  . 
Maple,  red  
sugar. 

Oak,  post  
red  
swamp  white 
tanbark.  .  .  . 

white  
it 

tt 

yellow.  . 

tt 

Osage  orange  
Sycamore  
Tupelo.  . 

THE    MECHANICAL    PROPERTIES    OF   WOOD 
TABLE  XIV.— Continued 


57 


Specific  gravity  oven- 
dry,  based  on 

Shrinkage  from  green  to  oven- 
dry  condition 

Mois- 

COMMON NAME 

ture 

OF   SPECIES 

con- 

Volume 

Volume 

In 

tent 

when 

\vhen 

volume 

Radial 

Tangential 

green 

oven-dry 

Conifers 

Per  cent 

Per  cent         Per  cent 

Per  cent 

Arborvitse  

55 

.293 

.315 

7.0               2.1 

4.9 

Cedar,  incense.  .  .  . 

80 

.363 

Cypress,  bald  .... 

79 

.452 

.513 

ii'.s 

3^8 

'Q.O 

Fir,  alpine  

47 

.306 

.321 

9.0 

2.5 

7.1 

amabilis  

117 

.383 

Douglas  

32 

.418 

'^458 

10*9 

3^7 

6.6 

white  

156 

.350 

.437 

10.2 

3.4 

7.0 

Hemlock  (east.)  .  . 

129 

.340 

.394 

9.2 

2.3 

5.0 

Pine,  lodgepole.  .  . 

44 

.370 

.415 

11.3 

4.2 

7.1 

a. 

58 

.371 

.407 

10.1 

3.6 

5.9 

longleaf  .  .  .  . 

63 

.528 

.599 

12.8 

6.0 

7.6 

red  or  Nor.  . 

54 

.440 

.507 

11.5 

4.5 

7.2 

shortleaf  .  .  .  . 

52 

.447 

sugar  

123 

.360 

.386 

8'4 

2.'9 

5.6 

west,  yellow 

98 

.353 

.395 

9.2 

4.1 

6.4 

«          « 

125 

.377 

.433 

11.5 

4.3 

7.3 

«          « 

93 

.391 

.435 

9.9 

3.8 

5.8 

white  

74 

.363 

.391  ' 

7.8 

2.2 

5.9 

Redwood 

81 

334 

it 

69 

.366 

Spruce, 

Engelmann  .... 

45 

.325 

359 

10.5 

3.7 

6.9 

« 

156 

.299 

.335 

10.3 

3.0 

6.2 

red  ;;:; 

31 

.396 

white  

41 

.318 

Tamarack  

52 

.491           .558 

13  '6           3.7 

'7A 

For  scientific  purposes  the  density  of  "  oven-dry  "  wood  is  used; 
that  is,  the  wood  is  dried  in  an  oven  at  a  temperature  of  100°  C. 
(212°  F.)  until  a  constant  weight  is  attained.  For  commercial 
purposes  the  weight  or  density  of  air-dry  or  "  shipping-dry  "  wood 
is  used.  This  is  usually  expressed  in  pounds  per  thousand  board 
feet,  a  board  foot  being  considered  as  one-twelfth  of  a  cubic  foot. 
Wood  shrinks  greatly  in  drying  from  the  green  to  the  oven-dry 
condition.  (See  Table  XIV.)  Consequently  a  block  of  wood  meas- 
uring a  cubic  foot  when  green  will  measure  considerably  less  when 
oven-dry.  It  follows  that  the  density  of  oven-dry  wood  does  not 
represent  the  weight  of  the  dry  wood  substance  in  a  cubic  foot  of 
green  wood.  In  other  words,  it  is  not  the  weight  of  a  cubic  foot 


58        THE  MECHANICAL  PROPERTIES  OF  WOOD 

of  green  wood  minus  the  weight  of  the  water  which  it  contains. 
Since  the  latter  is  often  a  more  convenient  figure  to  use  and  much 
easier  to  obtain  than  the  weight  of  oven-dry  wood,  it  is  commonly 
expressed  in  tables  of  "  specific  gravity  or  density  of  dry  wood." 
This  weight  divided  by  62.43  gives  the  specific  gravity  per  green 
volume.  It  is  purely  a  fictitious  quantity.  To  convert  this  figure 
into  actual  density  or  specific  gravity  of  the  dry  wood,  it  is  neces- 
sary to  know  the  amount  of  shrinkage  in  volume.  If  S  is  the  per- 
centage of  shrinkage  from  the  green  to  the  oven-dry  condition, 
based  on  the  green  volume;  D,  the  density  of  the  dry  wood  per 
cubic  foot  while  green;  and  d  the  actual  density  of  oven-dry  wood, 


This  relation  becomes  clearer  from  the  following  analysis: 
Taking  V  and  W  as  the  volume  and  weight,  respectively,  when 
green,  and  v  and  w  as  the  corresponding  volume  and  weight  when 

oven-dry,    then,    d  =  —  ;    D  =  ^-;   S  =  —  y—   X  100,   and   s  = 

V  —  v 

—  f  —  X  100,  in  which  S  is  the  percentage  of  shrinkage  from  the 

green  to  the  oven-dry  condition,  based  on  the  green  volume,  and 
s  the  same  based  on  the  oven-dry  volume. 

In  tables  of  specific  gravity  or  density  of  wood  it  should  always 
be  stated  whether  the  dry  weight  per  unit  of  volume  when  green 
or  the  dry  weight  per  unit  of  volume  when  dry  is  intended,  since 
the  shrinkage  in  volume  may  vary  from  6  to  50  per  cent,  though 
in  conifers  it  is  usually  about  10  per  cent,  and  in  hardwoods 
nearer  15  per  cent.  (See  Table  XIV.) 

COLOR 

In  species  which  show  a  distinct  difference  between  heartwood 
and  sapwood  the  natural  color  of  heartwood  is  invariably  darker 
than  that  of  the  sapwood,  and  very  frequently  the  contrast  is 
conspicuous.  This  is  produced  by  deposits  in  the  heartwood  of 
various  materials  resulting  from  the  process  of  growth,  increased 
possibly  by  oxidation  and  other  chemical  changes,  which  usually 
have  little  or  no  appreciable  effect  on  the  mechanical  properties  of 


THE  MECHANICAL  PROPERTIES  OF  WOOD        59 

the  wood.  (See  Heartwood  and  Sapwood.)  Some  experiments  * 
on  very  resinous  longleaf  pine  specimens,  however,  indicate  an  in- 
crease in  strength.  This  is  due  to  the  resin  which  increases  the 
strength  when  dry.  Spruce  impregnated  with  crude  resin  and 
dried  is  greatly  increased  in  strength  thereby. 

Since  the  late  wood  of  a  growth  ring  is  usually  darker  in  color 
than  the  early  wood,  this  fact  may  be  used  in  judging  the  density, 
and  therefore  the  hardness  and  strength  of  the  material.  This  is 
particularly  the  case  with  coniferous  woods.  In  ring-porous  woods 
the  vessels  of  the  early  wood  not  infrequently  appear  on  a  finished 
surface  as  darker  than  the  denser  late  wood,  though  on  cross  sec- 
tions of  heartwood  the  reverse  is  commonly  true.  Except  in  the 
manner  just  stated  the  color  of  wood  is  no  indication  of  strength. 

Abnormal  discoloration  of  wood  often  denotes  a  diseased  con- 
dition, indicating  unsoundness.  The  black  check  in  western  hem- 
lock is  the  result  of  insect  attacks,  f  The  reddish-brown  streaks 
so  common  in  hickory  and  certain  other  woods  are  mostly  the 
result  of  injury  by  birds.  J  The  discoloration  is  merely  an  indi- 
cation of  an  injury,  and  in  all  probability  does  not  of  itself  affect 
the  properties  of  the  wood.  Certain  rot-producing  fungi  impart 
to  wood  characteristic  colors  which  thus  become  criterions  of 
weakness.  Ordinary  sap-staining  is  due  to  fungous  growth,  but 
does  not  necessarily  produce  a  weakening  effect. § 

CROSS    GRAIN 

Cross  grain  is  a  very  common  defect  in  timber.  One  form  of 
it  is  produced  in  lumber  by  the  method  of  sawing  and  has  no 
reference  to  the  natural  arrangement  of  the  wood  elements.  Thus 


*  Bui.  70,  U.  S.  Forest  Service,  p.  92;  also  p.  126,  appendix. 

f  See  Burke,  H.  E. :  Black  check  in  western  hemlock.  Cir.  No.  61,  U. 
S.  Bu.  Entomology,  1905. 

|  See  McAtee,  W.  L.:  Woodpeckers  in  relation  to  trees  and  wood  prod- 
ucts. Bui.  No.  39,  U.  S.  Biol.  Survey,  1911. 

§  See  Von  Schrenck,  Hermann:  The  "bluing"  and  the  "red  rot"  of  the 
western  yellow  pine,  with  special  reference  to  the  Black  Hills  forest  reserve. 
Bui.  No.  36,  U.  S.  Bu.  Plant  Industry,  Washington,  1903,  pp.  13-14. 

Weiss,  Howard,  and  Barnum,  Charles  T. :  The  prevention  of  sapstain  in 
lumber.  Cir.  192,  U.  S.  Forest  Service,  Washington,  1911,  pp.  16-17. 


60  THE    MECHANICAL    PROPERTIES    OF   WOOD 

if  the  plane  of  the  saw  is  not  approximately  parallel  to  the  axis  of 
the  log  the  grain  of  the  lumber  cut  is  not  parallel  to  the  edges  and 
is  termed  diagonal.  This  is  likely  to  occur  where  the  logs  have 
considerable  taper,  and  in  this  case  may  be  produced  if  sawed 
parallel  to  the  axis  of  growth  instead  of  parallel  to  the  growth  rings. 

Lumber  and  timber  with  diagonal  grain  is  always  weaker  than 
straight-grained  material,  the  extent  of  the  defect  varying  with 
the  degree  of  the  angle  the  fibres  make  with  the  axis  of  the  stick. 
In  the  vicinity  of  large  knots  the  grain  is  likely  to  be  cross. 
The  defect  is  most  serious  where  wood  is  subjected  to  flexure, 
as  in  beams. 

Spiral  grain  is  a  very  common  defect  in  a  tree,  and  when 
excessive  renders  the  timber  valueless  for  use  except  in  the 
round.  It  is  produced  by  the  arrangement  of  the  wood  fibres 
in  a  spiral  direction  about  the  axis  instead  of  exactly  vertical. 
Timber  with  spiral  grain  is  also  known  as  "  torse  wood."  Spiral 
grain  usually  cannot  be  detected  by  casual  inspection  of  a 
stick,  since  it  does  not  show  in  the  so-called  visible  grain  of  the 
wood,  by  which  is  commonly  meant  a  sectional  view  of  the  annual 
rings  of  growth  cut  longitudinally.  It  is  accordingly  very  easy  to 
allow  spiral-grained  material  to  pass  inspection,  thereby  introduc- 
ing an  element  of  weakness  in  a  structure. 

There  are  methods  for  readily  detecting  spiral  grain.  The 
simplest  is  that  of  splitting  a  small  piece  radially.  It  is  necessary, 
of  course,  that  the  split  be  radial,  that  is,  in  a  plane  passing  through 
the  axis  of  the  log,  and  not  tangent ially.  In  the  latter  case  it  is 
quite  probable  that  the  wood  would  split  straight,  the  line  of 
cleavage  being  between  the  growth  rings. 

In  inspection,  the  elements  to  examine  are  the  rays.  In  the 
case  of  oak  and  certain  other  hardwoods  these  rays  are  so  large 
that  they  are  readily  seen  not  only  on  a  radial  surface,  but  on 
the  tangential  as  well.  On  the  former  they  appear  as  flakes,  on  the 
latter  as  short  lines.  Since  these  rays  are  between  the  fibres  it 
naturally  follows  that  they  will  be  vertical  or  inclined  according  as 
the  tree  is  straight-grained  or  spiral-grained.  While  they  are  not 
conspicuous  in  the  softwoods,  they  can  be  seen  upon  close  scrutiny, 
and  particularly  so  if  a  small  hand  magnifier  is  used. 

When  wood  has  begun  to  dry  and  check  it  is  very  easy  to  see 


THE  MECHANICAL  PROPERTIES  OF  WOOD        61 

whether  or  not  it  is  straight-  or  spiral-grained,  since  the  checks 
will  for  the  most  part  follow  along  the  rays.  If  one  examines  a  row 
of  telephone  poles,  for  example,  he  will  probably  find  that  most  of 
them  have  checks  running  spirally  around  them.  If  boards  were 
sawed  from  such  a  pole  after  it  was  badly  checked  they  would  fall 
to  pieces  of  their  own  weight.  The  only  way  to  get  straight 
material  would  be  to  split  it  out. 

It  is  for  this  reason  that  split  billets  and  squares  are  stronger 
than  most  sawed  material.  The  presence  of  the  spiral  grain  has 
little,  if  any,  effect  on  the  timber  when  it  is  used  in  the  round,  but 
in  sawed  material  the  greater  the  pitch  of  the  spiral  the  greater 
is  the  defect. 

KNOTS 

Knots  are  portions  of  branches  included  in  the  wood  of  the 
stem  or  larger  branch.  Branches  originate  as  a  rule  from  the 
central  axis  of  a  stem,  and  while  living  increase  in  size  by  the 
addition  of  annual  woody  layers  which  are  a  continuation  of  those 
of  the  stem.  The  included  portion  is  irregularly  conical  in  shape 
with  the  tip  at  the  pith.  The  direction  of  the  fibre  is  at  right 
angles  or  oblique  to  the  grain  of  the  stem,  thus  producing  local 
cross  grain. 

During  the  development  of  a  tree  most  of  the  limbs,  especially 
the  lower  ones,  die,  but  persist  for  a  time — often  for  years.  Sub- 
sequent layers  of  growth  of  the  stem  are  no  longer  intimately 
joined  with  the  dead  limb,  but  are  laid  around  it.  Hence  dead 
branches  produce  knots  which  are  nothing  more  than  pegs  in  a 
hole,  and  likely  to  drop  out  after  the  tree  has  been  sawed  into  lum- 
ber. In  grading  lumber  and  structural  timber,  knots  are  classified 
according  to  their  form,  size,  soundness,  and  the  firmness  with 
which  they  are  held  in  place.* 

Knots  materially  affect  checking  and  warping,  ease  in  working, 
and  cleavability  of  timber.  They  are  defects  which  weaken  timber 
and  depreciate  its  value  for  structural  purposes  where  strength  is 
an  important  consideration.  The  weakening  effect  is  much  more 

*  See  Standard  classification  of  structural  timber.  Yearbook  Am.  Soc. 
for  Testing  Materials,  1913,  pp.  300-303.  Contains  three  plates  showing 
standard  defects. 


62  THE    MECHANICAL    PROPERTIES    OF   WOOD 

serious  where  timber  is  subjected  to  bending  and  tension  than 
where  under  compression.  The  extent  to  which  knots  affect  the 
strength  of  a  beam  depends  upon  their  position,  size,  number, 
direction  of  fibre,  and  condition.  A  knot  on  the  upper  side  is 
compressed,  while  one  on  the  lower  side  is  subjected  to  tension. 
The  knot,  especially  (as  is  often  the  case)  if  there  is  a  season 
check  in  it,  offers  little  resistance  to  this  tensile  stress.  Small 
knots,  however,  may  be  so  located  in  a  beam  along  the  neutral 
plane  as  actually  to  increase  the  strength  by  tending  to  prevent 
longitudinal  shearing.  Knots  in  a  board  or  plank  are  least  injuri- 
ous when  they  extend  through  it  at  right  angles  to  its  broadest 
surface.  Knots  which  occur  near  the  ends  of  a  beam  do  not 
weaken  it.  Sound  knots  which  occur  in  the  central  portion  one- 
fourth  the  height  of  the  beam  from  either  edge  are  not  serious 
defects. 

Extensive  experiments  by  the  U.  S.  Forest  Service  *  indicate 
the  following  effects  of  knots  on  structural  timbers: 

(1)  Knots  do  not  materially  influence  the  stiffness  of  struc- 
tural timber. 

(2)  Only  defects  of  the  most  serious  character  affect  the  elastic 
limit  of  beams.    Stiffness  and  elastic  strength  are  more  dependent 
upon  the  quality  of  the  wood  fibre  than  upon  defects  in  the  beam. 

(3)  The  effect  of  knots  is  to  reduce  the  difference  between  the 
fibre  stress  at  elastic  limit  and  the  modulus  of  rupture  of  beams. 
The  breaking  strength  is  very  susceptible  to  defects. 

(4)  Sound  knots  do  not  weaken  wood  when  subject  to  com- 
pression parallel  to  the  grain,  f 

FROST    SPLITS 

A  common  defect  in  standing  timber  results  from  radial  splits 
which  extend  inward  from  the  periphery  of  the  tree,  and  almost, 
if  not  always,  near  the  base.  It  is  most  common  in  trees  which 
split  readily,  and  those  with  large  rays  and  thin  bark.  The 


*  Bui.  108,  pp.  52    et  seq. 

f  Bui.  115,  U.  S.  Forest  Service:    Mechanical  properties  of  western  horn- 
lock,  p.  20. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        63 

primary  cause  of  the  splitting  is  frost,  and  various  theories  have 
been  advanced  to  explain  the  action. 

R.  Hartig  *  believes  that  freezing  forces  out  a  part  of  the 
imbibition  water  of  the  cell  walls,  thereby  causing  the  wood  to 
shrink,  and  if  the  interior  layers  have  not  yet  been  cooled,  tangen- 
tial strains  arise  which  finally  produce  radial  clefts. 

Another  theory  holds  that  the  water  is  not  driven  out  of  the 
cell  walls,  but  that  difference  in  temperature  conditions  of  inner 
and  outer  layers  is  itself  sufficient  to  set  up  the  strains,  resulting 
in  splitting.  An  air  temperature  of  14°  F.  or  less  is  considered 
necessary  to  produce  frost  splits. 

A  still  more  recent  theory  is  that  of  Busse  f  who  considers 
the  mechanical  action  of  the  wind  a  very  important  factor.  He 
observed:  (a)  Frost  splits  sometimes  occur  at  higher  temper- 
atures than  14°  F.  (6)  Most  splits  take  place  shortly  before 
sunrise,  i.e.,  at  the  time  of  lowest  air  and  soil  temperature;  they 
are  never  heard  to  take  place  at  noon,  afternoon,  or  evening, 
(c)  They  always  occur  between  two  roots  or  between  the  collars 
of  two  roots,  (d)  They  are  most  frequent  in  old,  stout-rooted, 
broad-crowned  trees;  in  younger  stands  it  is  always  the  stoutest 
members  that  are  found  with  frost  splits,  while  in  quite  young 
stands  they  are  altogether  absent,  (e)  Trees  on  wet  sites  are 
most  liable  to  splits,  due  to  difference  in  wood  structure,  just  as 
difference  in  wood  structure  makes  different  species  vary  in  this 
regard.  (/)  Frost  splits  are  most  numerous  less  than  three  feet 
above  the  ground. 

When  a  tree  is  swayed  by  the  wind  the  roots  are  counteracting 
forces,  and  the  wood  fibres  are  tested  in  tension  and  compression 
by  the  opposing  forces;  where  the  roots  exercise  tension  stresses 
most  effectively  the  effect  of  compression  stresses  is  at  a  minimum; 
only  where  the  pressure  is  in  excess  of  the  tension,  i.e.,  between 
the  roots,  can  a  separation  of  the  fibre  result.  Hence,  when  by 
frost  a  tension  on  the  entire  periphery  is  established,  and  the 


*  Hartig,  R.:  The  diseases  of  trees  (trans,  by  Somerville  and  Ward),  Lon- 
don and  New  York,  1894,  pp.  282-294. 

f  Busse,  W.:  Frost-,  Ring-  und  Kernrisse.  Forstwiss.  Centralb.,  XXXII, 
2,  1910,  pp.  74-81. 


64        THE  MECHANICAL  PROPERTIES  OF  WOOD 

wind  localizes  additional  strains,  failure  occurs.  The  stronger  the 
compression  and  tension,  the  severer  the  strains  and  the  oftener 
failures  occur.  The  occurrence  of  reports  of  frost  splits  on  wind- 
still  days  is  believed  by  Busse  to  be  due  to  the  opening  of  old  frost 
splits  where  the  tension  produced  by  the  frost  alone  is  sufficient. 
Frost  splits  may  heal  over  temporarily,  but  usually  open  up 
again  during  the  following  winter.  The  presence  of  old  splits  is 
often  indicated  by  a  ridge  of  callous,  the  result  of  the  cambium's 
effort  to  occlude  the  wound.  Frost  splits  not  only  affect  the 
value  of  lumber,  but  also  afford  an  entrance  into  the  living  tree  for 
disease  and  decay. 

SHAKES,    GALLS,    PITCH    POCKETS 

Heart  shake  occurs  in  nearly  all  overmature  timber,  being 
more  frequent  in  hardwoods  (especially  oak)  than  in  conifers. 
In  typical  heart  shake  the  centre  of  the  bole  shows  indications  of 
becoming  hollow  and  radial  clefts  of  varying  size  extend  outward 
from  the  pith,  being  widest  inward.  It  frequently  affects  only 
the  butt  log,  but  may  extend  to  the  entire  bole  and  even  the 
larger  branches.  It  usually  results  from  a  shrinkage  of  the  heart- 
wood  due  probably  to  chemical  changes  in  the  wood. 

When  it  consists  of  a  single  cleft  extending  across  the  pith  it 
is  termed  simple  heart  shake.  Shake  of  this  character  in  straight- 
grained  trees  affects  only  one  or  two  central  boards  when  cut 
into  lumber,  but  in  spiral-grained  timber  the  damage  is  much 
greater.  When  shake  consists  of  several  radial  clefts  it  is  termed 
star  shake.  In  some  instances  one  or  more  of  these  clefts  may 
extend  nearly  to  the  bark.  In  felled  or  converted  timber  clefts 
due  to  heart  shake  may  be  distinguished  from  seasoning  cracks 
by  the  darker  color  of  the  exposed  surfaces.  Such  clefts,  however, 
tend  to  open  up  more  and  more  as  the  timber  seasons. 

Cup  or  ring  shake  results  from  the  pulling  apart  of  two  or 
more  growth  rings.  It  is  one  of  the  most  serious  defects  to  which 
sound  timber  is  subject,  as  it  seriously  reduces  the  technical  prop- 
erties of  wood.  It  is  very  common  in  sycamore  and  in  western 
larch,  particularly  in  the  butt  portion.  Its  occurrence  is  most 
frequent  at  the  junction  of  two  growth  layers  of  very  unequal 


THE  MECHANICAL  PROPERTIES  OF  WOOD        65 

thickness.  Consequently  it  is  likely  to  occur  in  trees  which  have 
grown  slowly  for  a  time,  then  abruptly  increased,  due  to  improved 
conditions  of  light  and  food,  as  in  thinning.  Old  timber  is  more 
subject  to  it  than  young  trees.  The  damage  is  largely  confined 
to  the  butt  log.  Cup  shake  is  often  associated  with  other  forms 
of  shake,  and  not  infrequently  shows  traces  of  decay. 

The  causes  of  cup  shake  are  uncertain.  The  swaying  action 
of  the  wind  may  result  in  shearing  apart  the  growth  layers, 
especially  in  trees  growing  in  exposed  places.  Frost  may  in  some 
instances  be  responsible  for  cup  shake  or  at  least  a  contributing 
factor,  although  trees  growing  in  regions  free  from  frost  often 
have  ring  shake.  Shrinkage  of  the  heartwood  may  be  concentric 
as  well  as  radial  in  its  action,  thus  producing  cup  shake  instead  of, 
or  in  connection  with,  heart  shake. 

A  local  defect  somewhat  similar  in  effect  to  cup  shake  is  known 
as  rind  gall.  If  the  cambium  layer  is  exposed  by  the  removal  of 
the  entire  bark  or  rind  it  will  die.  Subsequent  growth  over  the 
damaged  portion  does  not  cohere  with  the  wood  previously 
formed  by  the  old  cambium.  The  defect  resulting  is  termed  rind 
gall.  The  most  common  causes  of  it  are  fire,  gnawing,  blazing, 
chipping,  sun  scald,  lightning,  and  abrasions. 

Heart  break  i$  a  term  applied  to  areas  of  compression  failure 
along  the  grain  found  in  occasional  logs.  Sometimes  these  breaks 
are  invisible  until  the  wood  is  manufactured  into  the  finished 
article.  The  occurrence  of  this  defect  is  mostly  limited  to  the 
dense  hardwoods,  such  as  hickory  and  to  heavy  tropical  species. 
It  is  the  source  of  considerable  loss  in  the  fancy  veneer  industry, 
as  the  veneer  from  valuable  logs  so  affected  drops  to  pieces. 

The  cause  of  heart  break  is  not  positively  known.  It  is  highly 
probable,  however,  that  when  the  tree  is  felled  the  trunk  strikes 
across  a  rock  or  another  log,  and  the  impact  causes  actual  failure 
in  the  log  as  in  a  beam. 

Resin  or  pitch  pockets  are  of  common  occurrence  in  the  wood 
of  larch,  spruce,  fir,  and  especially  of  longleaf  and  other  hard 
pines.  They  are  due  to  accumulations  of  resin  in  openings  be- 
tween adjacent  layers  of  growth.  They  are  more  frequent  in  trees 
growing  alone  than  in  those  of  dense  stands.  The  pockets  are 
usually  a  few  inches  in  greatest  dimension  and  affect  only  one  or 


66         THE  MECHANICAL  PROPERTIES  OF  WOOD 

two  growth  layers.  They  are  hidden  until  exposed  by  the  saw, 
rendering  it  impossible  to  cut  lumber  with  reference  to  their 
position.  Often  several  boards  are  damaged  by  a  single  pocket. 
In  grading  lumber,  pitch  pockets  are  classified  as  small,  standard, 
and  large,  depending  upon  their  width  and  length. 

INSECT    INJURIES* 

The  larvae  of  many  insects  are  destructive  to  wood.  Some 
attack  the  wood  of  living  trees,  others  only  that  of  felled  or  con- 
verted material.  Every  hole  breaks  the  continuity  of  the  fibres 
and  impairs  the  strength,  and  if  there  are  very  many  of  them  the 
material  may  be  ruined  for  all  purposes  where  strength  is 
required. 

Some  of  the  most  common  insects  attacking  the  wood  of  living 
trees  are  the  oak  timber  worm,  the  chestnut  timber  worm,  car- 
penter worms,  ambrosia  beetles,  the  locust  borer,  turpentine 
beetles  and  turpentine  borers,  and  the  white  pine  weevil. 

The  insect  injuries  to  forest  products  may  be  classed  according 
to  the  stage  of  manufacture  of  the  material.  Thus  round  timber 
with  the  bark  on,  such  as  poles,  posts,  mine  props,  and  sawlogs, 
is  subject  to  serious  damage  by  the  same  class  of  insects  as  those 
mentioned  above,  particularly  by  the  round-headed  borers,  tim- 
ber worms,  and  ambrosia  beetles.  Manufactured  unseasoned 
products  are  subject  to  damage  from  ambrosia  beetles  and  other 
wood  borers.  Seasoned  hardwood  lumber  of  all  kinds,  rough 
handles,  wagon  stock,  etc.,  made  partially  or  entirely  of  sapwood, 
are  often  reduced  in  value  from  10  to  90  per  cent  by  a  class  of 
insects  known  as  powder-post  beetles.  Finished  hardwood  prod- 
ucts such  as  handles,  wagon,  carriage  and  machinery  stock,  espe- 
cially if  ash  or  hickory,  are  often  destroyed  by  the  powder-post 
beetles.  Construction  timbers  in  buildings,  bridges  and  trestles, 
cross-ties,  poles,  mine  props,  fence  posts,  etc.,  are  sometimes 
seriously  injured  by  wood-boring  larvae,  termites,  black  ants, 
carpenter  bees,  and  powder-post  beetles,  and  sometimes  reduced 

*  For  detailed  information  regarding  insect  injuries,  the  reader  is  referred 
to  the  various  publications  of  the  U.  S.  Bureau  of  Entomology,  Washington, 
D.  C. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        67 

in  value  from  10  to  100  per  cent.     In  tropical  countries  termites 
are  a  very  serious  pest  in  this  respect. 

MARINE  WOOD-BORER  INJURIES 

Vast  amounts  of  timber  used  for  piles  in  wharves  and  other 
marine  structures  are  constantly  being  destroyed  or  seriously 
injured  by  marine  borers.  Almost  invariably  they  are  confined  to 
salt  water,  and  all  the  woods  commonly  used  for  piling  are  subject 
to  their  attacks.  There  are  two  genera  of  mollusks,  Xylotrya  and 
Teredo,  and  three  of  crustaceans,  Limnoria,  Chelura,  and  Sphce- 
roma,  that  do  serious  damage  in  many  places  along  both  the 
Atlantic  and  Pacific  coasts. 

These  mollusks,  which  are  popularly  known  as  "  shipworms," 
are  much  alike  in  structure  and  mode  of  life.  They  attack  the 
exposed  surface  of  the  wood  and  immediately  begin  to  bore.  The 
tunnels,  often  as  large  as  a  lead  pencil,  extend  usually  in  a  longi- 
tudinal direction  and  follow  a  very  irregular,  tangled  course. 
Hard  woods  are  apparently  penetrated  as  readily  as  soft  woods, 
though  in  the  same  timber  the  softer  parts  are  preferred.  The 
food  consists  of  infusoria  and  is  not  obtained  from  the  wood  sub- 
stance. The  sole  object  of  boring  into  the  wood  is  to  obtain 
shelter. 

Although  shipworms  can  live  in  cold  water  they  thrive  best 
and  are  most  destructive  in  warm  water.  The  length  of  time 
required  to  destroy  an  average  barked,  unprotected  pine  pile  on 
the  Atlantic  coast  south  from  Chesapeake  Bay  and  along  the 
entire  Pacific  coast  varies  from  but  one  to  three  years. 

Of  the  crustacean  borers,  Limnoria,  or  the  "  wood  louse,  Is 
the  only  one  of  great  importance,  although  Sphceroma  is  reported 
destructive  in  places.  Limnoria  is  about  the  size  of  a  grain  of 
rice  and  tunnels  into  the  wood  for  both  food  and  shelter.  The 
galleries  extend  inward  radially,  side  by  side,  in  countless  numbers, 
to  the  depth  of  about  one-half  inch.  The  thin  wood  partitions 
remaining  are  destroyed  by  wave  action,  so  that  a  fresh  surface 
is  exposed  to  attack.  Both  hard  and  soft  woods  are  damaged,  but 
the  rate  is  faster  in  the  soft  woods  or  softer  portions  of  a  wood. 

Timbers  seriously  attacked  by  marine  borers  are  badly  weak- 


68        THE  MECHANICAL  PROPERTIES  OF  WOOD 

ened  or  completely  destroyed.  If  the  original  strength  of  the 
material  is  to  be  preserved  it  is  necessary  to  protect  the  wood 
from  the  borers.  This  is  sometimes  accomplished  by  proper 
injection  of  creosote  oil,  and  more  or  less  successfully  by  the  use 
of  various  kinds  of  external  coatings.*  No  treatment,  however, 
has  proved  entirely  satisfactory. 

FUNGOUS    INJURIES  f 

Fungi  are  responsible  for  almost  all  decay  of  wood.  So  far  as 
known,  all  decay  is  produced  by  living  organisms,  either  fungi  or 
bacteria.  Some  species  attack  living  trees,  sometimes  killing  them, 
or  making  them  hollow,  or  in  the  case  of  pecky  cypress  and  incense 
cedar  filling  the  wood  with  galleries  like  those  of  boring  insects. 
A  much  larger  variety  work  only  in  felled  or  dead  wood,  even 
after  it  is  placed  in  buildings  or  manufactured  articles.  In  any 
case  the  process  of  destruction  is  the  same.  The  mycelial  threads 
penetrate  the  walls  of  the  cells  in  search  of  food,  which  they  find 
either  in  the  cell  contents  (starches,  sugars,  etc.),  or  in  the  cell 
wall  itself.  The  breaking  down  of  the  cell  walls  through  the 
chemical  action  of  so-called  "  enzymes  "  secreted  by  the  fungi 
follows,  and  the  eventual  product  is  a  rotten,  moist  substance 
crumbling  readily  under  the  slightest  pressure.  Some  species  re- 
move the  ligneous  matter  and  leave  almost  pure  cellulose,  which 
is  white,  like  cotton;  others  dissolve  the  cellulose,  leaving  a  brittle, 
dark  brown  mass  of  ligno-cellulose.  Fungi  (such  as  the  bluing 
fungus)  which  merely  stain  wood  usually  do  not  affect  its  mechan- 
ical properties  unless  the  attacks  are  excessive. 

It  is  evident,  then,  that  the  action  of  rot-causing  fungi  is  to 
decrease  the  strength  of  wood,  rendering  it  unsound,  brittle,  and 


*  See  Smith,  C.  Stowell:  Preservation  of  piling  against  marine  wood 
borers.  Cir.  128,  U.  S.  Forest  Service,  1908,  pp.  15. 

f  See  Von  Schrenck,  H.:  The  decay  of  timber  and  methods  of  preventing 
it.  Bui.  14,  U.  S.  Bu.  Plant  Industry,  Washington,  D.  C.,  1902.  Also 
Buls.  32,  114,  214,  266. 

Meinecke,  E.  P.:  Forest  tree  diseases  common  in  California  and  Nevada, 
U.  S.  Forest  Service,  Washington,  D.  C.,  1914. 

Hartig,  R.:    The  diseases  of  trees.     London  and  New  York,  1894. 


THE   MECHANICAL   PROPERTIES    OF   WOOD  69 

dangerous  to  use.  The  most  dangerous  kinds  are  the  so-called 
"  dry-rot  "  fungi  which  work  in  many  kinds  of  lumber  after  it  is 
placed  in  the  buildings.  They  are  particularly  to  be  dreaded 
because  unseen,  working  as  they  do  within  the  walls  or  inside  of 
casings.  Several  serious  wrecks  of  large  buildings  have  been 
attributed  to  this  cause.  It  is  stated  *  that  in  the  three  years 
(1911-1913)  more  than  $100,000  was  required  to  repair  damage 
due  to  dry  rot. 

Dry  rot  develops  best  at  75°  F.  and  is  said  to  be  killed  by  a 
temperature  of  110°  F.  f  Fully  70  per  cent  humidity  is  necessary 
in  the  air  in  which  a  timber  is  surrounded  for  the  growth  of  this 
fungus,  and  probably  the  wood  must  be  quite  near  its  fibre  satura- 
tion condition.  Nevertheless  Merulius  lacrymans  (one  of  the  most 
important  species)  has  been  found  to  live  four  years  and  eight 
months  in  a  dry  condition.  J  Thorough  kiln-drying  will  kill  this 
fungus,  but  will  not  prevent  its  redevelopment.  Antiseptic  treat- 
ment, such  as  creosoting,  is  the  best  prevention. 

All  fungi  require  moisture  and  air  §  for  their  growth.  De- 
prived of  either  of  these  the  fungus  dies  or  ceases  to  develop. 
Just  what  degree  of  moisture  in  wood  is  necessary  for  the  "  dry- 
rot  "  fungus  has  not  been  determined,  but  it  is  evidently  consid- 
erably above  that  of  thoroughly  air-dry  timber,  probably  more 
than  15  per  cent  moisture.  Hence  the  importance  of  free  circu- 
lation of  air  about  all  timbers  in  a  building. 

Warmth  is  also  conducive  to  the  growth  of  fungi,  the  most 
favorable  temperature  being  about  90°  F.  They  cannot  grow  in 
extreme  cold,  although  no  degree  of  cold  such  as  occurs  naturally 
will  kill  them.  On  the  other  hand,  high  temperature  will  kill 
them,  but  the  spores  may  survive  even  the  boiling  temperature. 
Mould  fungus  has  been  observed  to  develop  rapidly  at  130°  F.  in 
a  dry  kiln  in  moist  air,  a  condition  under  which  an  animal  cannot 


*  Dry  rot  in  factory  timbers,  by  Inspection  Dept.  Associated  Factory 
Mutual  Fire  Insurance  Cos.,  31  Milk  Street,  Boston,  1913. 

fFalck,  Richard:  Die  Meruliusfalile  des  Bauholzes,  Hausschwammfor- 
schungen,  6.  Heft.,  Jena,  1912. 

J  Mez,  Carl:  Der  Hausschwamm.     Dresden,  1908,  p.  63. 

§A  culture  of  fungus  placed  in  a  glass  jar  and  the  air  pumped  out  ceases 
to  grow,  but  will  start  again  as  soon  as  oxygen  is  admitted. 


70        THE  MECHANICAL  PROPERTIES  OF  WOOD 

live  more  than  a  few  minutes.    This  fungus  was  killed,  however, 
at  about  140°  or  145°  F.* 

The  fungus  (Endothia  parasitica  And.)  which  causes  the 
chestnut  blight  kills  the  trees  by  girdling  them  and  has  no  direct 
effect  upon  the  wood  save  possibly  the  four  or  five  growth  rings 
of  the  sap  wood,  t 

PARASITIC    PLANT   INJURIES. J 

The  most  common  of  the  higher  parasitic  plants  damaging 
timber  trees  are  mistletoes.  Many  species  of  deciduous  trees  are 
attacked  by  the  common  mistletoe  (Phoradendron  flavescens)'.  It 
is  very  prevalent  in  the  South  and  Southwest  and  when  present 
in  sufficient  quantity  does  considerable  damage.  There  is  also  a 
considerable  number  of  smaller  mistletoes  belonging  to  the  genus 
Razoumofskya  (Arceuthobium)  which  are  widely  distributed 
throughout  the  country,  and  several  of  them  are  common  on  conif- 
erous trees  in  the  Rocky  Mountains  and  along  the  Pacific  coast. 

One  effect  of  the  common  mistletoe  is  the  formation  of  large 
swellings  or  tumors.  Often  the  entire  tree  may  become  stunted 
or  distorted.  The  western  mistletoe  is  most  common  on  the 
branches,  where  it  produces  "  witches'  broom."  It  frequently 
attacks  the  trunk  as  well,  and  boards  cut  from  such  trees  are 
filled  with  long,  radial  holes  which  seriously  damage  or  destroy 
the  value  of  the  timber  affected. 

LOCALITY    OF    GROWTH 

The  data  available  regarding  the  effect  of  the  locality  of 
growth  upon  the  properties  of  wood  are  not  sufficient  to  warrant 


*  Experiments  in  kiln-drying  Eucalyptus  in  Berkeley,  U.  S.  Forest  Service. 

t  See  Anderson,  Paul  J. :  The  morphology  and  life  history  of  the  chestnut 
blight  fungus.  Bui.  No.  7,  Penna.  Chestnut  Tree  Blight  Com.,  Harrisburg, 
1914,  p.  17. 

t  See  York,  Harlan  H.:  The  anatomy  and  some  of  the  biological  aspects 
of  the  "  American  mistletoe."  Bui.  120,  Sci.  Ser.  No.  13,  Univ.  of  Texas, 
Austin,  1909. 

Bray,  Wm.  L.:  The  mistletoe  pest  in  the  Southwest.  Bui.  166,  U.  S.  Bu. 
Plant  Ind.,  Washington,  1910. 

Meinecke,  E.  P.:  Forest  tree  diseases  common  in  California  and  Nevada. 
U.  S.  Forest  Service,  Washington,  1914,  pp.  54-58. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        71 

definite  conclusions.  The  subject  has,  however,  been  kept  in 
mind  in  many  of  the  U.  S.  Forest  Service  timber  tests  and  the 
following  quotations  are  assembled  from  various  reports: 

"  In  both  the  Cuban  and  longleaf  pine  the  locality  where 
grown  appears  to  have  but  little  influence  on  weight  or  strength, 
and  there  is  no  reason  to  believe  that  the  longleaf  pine  from  one 
State  is  better  than  that  from  any  other,  since  such  variations  as 
are  claimed  can  be  found  on  any  40-acre  lot  of  timber  in  any  State. 
But  with  loblolly  and  still  more  with  shortleaf  this  seems  not  to 
be  the  case.  Being  widely  distributed  over  many  localities  dif- 
ferent in  soil  and  climate,  the  growth  of  the  shortleaf  pine  seems 
materially  influenced  by. location.  The  wood  from  the  southern 
coast  and  gulf  region  and  even  Arkansas  is  generally  heavier  than 
the  wood  from  localities  farther  north.  Very  light  and  fine- 
grained wood  is  seldom  met  near  the  southern  limit  of  the  range, 
while  it  is  almost  the  rule  in  Missouri,  where  forms  resembling 
the  Norway  pine  are  by  no  means  rare.  The  loblolly,  occupying 
both  wet  and  dry  soils,  varies  accordingly."  Cir.  No.  12,  p.  6. 

"  .  .  .  It  is  clear  that  as  all  localities  have  their  heavy  and 
their  light  timber,  so  they  all  share  in  strong  and  weak,  hard  and 
soft  material,  and  the  difference  in  quality  of  material  is  evidently 
far  more  a  matter  of  individual  variation  than  of  soil  or  climate." 
Ibid.,  p.  22 

"  A  representative  committee  of  the  Carriage  Builders'  Asso- 
ciation had  publicly  declared  that  this  important  industry  could 
not  depend  upon  the  supplies  of  southern  timber,  as  the  oak 
grown  in  the  South  lacked  the  necessary  qualities  demanded  in 
carriage  construction.  Without  experiment  this  statement  could 
be  little  better  than  a  guess,  and  was  doubly  unwarranted,  since 
it  condemned  an  enormous  amount  of  material,  and  one  produced 
under  a  great  variety  of  conditions  and  by  at  least  a  dozen  species 
of  trees,  involving,  therefore,  a  complexity  of  problems  difficult 
enough  for  the  careful  investigator,  and  entirely  beyond  the  few 
unsystematic  observations  of  the  members  of  a  committee  on  a 
flying  trip  through  one  of  the  greatest  timber  regions  of  the  world. 
"  A  number  of  samples  were  at  once  collected  (part  of  them 
supplied  by  the  carriage  builders'  committee),  and  the  fallacy  of 
the  broad  statement  mentioned  was  fully  demonstrated  by  a  short 


72         THE  MECHANICAL  PROPERTIES  OF  WOOD 

series  of  tests  and  a  more  extensive  study  into  structure  and 
weight  of  these  materials.  From  these  tests  it  appears  that  pieces 
of  white  oak  from  Arkansas  excelled  well-selected  pieces  from 
Connecticut,  both  in  stiffness  and  endwise  compression  (the  two 
most  important  forms  of  resistance)."  Report  upon  the  forestry 
investigations  of  the  U.  S.  D.  A.  1877-1898,  p.  331.  See  also 
Rep.  of  Div.  of  For.,  1890,  p.  209. 

"  In  some  regions  there  are  many  small,  stunted  hickories, 
which  most  users  will  not  touch.  They  have  narrow  sap,  are 
likely  to  be  birdpecked,  and  show  very  slow  growth.  Yet  five  of 
these  trees  from  a  steep,  dry  south  slope  in  West  Virginia  had  an 
average  strength  fully  equal  to  that  of  the  pignut  from  the  better 
situation,  and  were  superior  in  toughness,  the  work  to  maximum 
load  being  36.8  as  against  31.2  for  pignut.  The  trees  had  about 
twice  as  many  rings  per  inch  as  others  from  better  situations. 

"  This,  however,  is  not  very  significant,  as  trees  of  the  same 
species,  age,  and  size,  growing  side  by  side  under  the  same  condi- 
tions of  soil  and  situation,  show  great  variation  in  their  technical 
value.  It  is  hard  to  account  for  this  difference,  but  it  seems  that 
trees  growing  in  wet  or  moist  situations  are  rather  inferior  to 
those  growing  on  fresher  soil;  also,  it  is  claimed  by  many  hickory 
users  that  the  wood  from  limestone  soils  is  superior  to  that  from 
sandy  soils. 

"  One  of  the  moot  questions  among  hickory  men  is  the  relative 
value  of  northern  and  southern  hickory.  The  impression  prevails 
that  southern  hickory  is  more  porous  and  brash  than  hickory  from 
the  north.  The  tests  .  .  .  indicate  that  southern  hickory  is  as 
tough  and  strong  as  northern  hickory  of  the  same  age.  But  the 
southern  hickories  have  a  greater  tendency  to  be  shaky,  and  this 
results  in  much  waste.  In  trees  from  southern  river  bottoms  the 
loss  through  shakes  and  grub-holes  in  many  cases  amounts  to  as 
much  as  50  per  cent. 

"It  is  clear,  therefore,  that  the  difference  in  northern  and 
southern  hickory  is  not  due  to  geographic  location,  but  rather  to 
the  character  of  timber  that  is  being  cut.  Nearly  all  of  that  from 
southern  river  bottoms  and  from  the  Cumberland  Mountains  is 
from  large,  old-growth  trees;  that  from  the  north  is  from  younger 
trees  which  are  grown  under  more  favorable  conditions,  and  it  is 


THE  MECHANICAL  PROPERTIES  OF  WOOD        73 

due  simply  to  the  greater  age  of  the  southern  trees  that  hickory 
from  that  region  is  lighter  and  more  brash  than  that  from  the 
north."  Bui.  80,  pp.  52-55. 


SEASON    OF    CUTTING 

It  is  generally  believed  that  winter-felled  timber  has  decided 
advantages  over  that  cut  at  other  seasons  of  the  year,  and  to  that 
cause  alone  are  frequently  ascribed  much  greater  durability,  less 
liability  to  check  and  split,  better  color,  and  even  increased  strength 
and  toughness.  The  conclusion  from  the  various  experiments  made 
on  the  subject  is  that  while  the  time  of  felling  may,  and  often  does, 
affect  the  properties  of  wood,  such  result  is  due  to  the  weather 
conditions  rather  than  to  the  condition  of  the  wood. 

There  are  two  phases  of  this  question.  One  is  concerned  with 
the  physiological  changes  which  might  take  place  during  the  year 
in  the  wood  of  a  living  tree.  The  other  deals  with  the  purely  phys- 
ical results  due  to  the  weather,  as  differences  in  temperature, 
humidity,  moisture,  and  other  features  to  be  mentioned  later. 

Those  who  adhere  to  the  first  view  maintain  that  wood  cut 
in  summer  is  quite  different  in  composition  from  that  cut  in  win- 
ter. One  opinion  is  that  in  summer  the  "  sap  is  up,"  while  in 
winter  it  is  "  down,"  consequently  winter-felled  timber  is  drier. 
A  variation  of  this  belief  is  that  in  summer  the  sap  contains  cer- 
tain chemicals  which  affect  the  properties  of  wood  and  does  not 
contain  them  in  winter.  Again  it  is  sometimes  asserted  that  wood 
is  actually  denser  in  winter  than  in  summer,  as  part  of  the  wood 
substance  is  dissolved  out  in  the  spring  and  used  for  plant  food, 
being  restored  in  the  fall. 

It  is  obvious  that  such  views  could  apply  only  to  sapwood, 
since  it  alone  is  in  living  condition  at  the  time  of  cutting.  Heart- 
wood  is  dead  wood  and  has  almost  no  function  in  the  existence,  of 
the  tree  other  than  the  purely  mechanical  one  of  support.  Heart- 
wood  does  undergo  changes,  but  they  are  gradual  and  almost 
entirely  independent  of  the  seasons. 

Sapwood  might  reasonably  be  expected  to  respond  to  seasonal 
changes,  and  to  some  extent  it  does.  Just  beneath  the  bark  there 
is  a  thin  layer  of  cells  which  during  the  growing  season  have  not 


74        THE  MECHANICAL  PROPERTIES  OF  WOOD 

attained  their  greatest  density.  With  the  exception  of  this  one 
annual  ring,  or  portion  of  one,  the  density  of  the  wood  substance 
of  the  sapwood  is  nearly  the  same  the  year  round.  Slight  varia- 
tions may  occur  due  to  impregnation  with  sugar  and  starch  in 
the  winter  and  its  dissolution  in  the  growing  season.  The  time  of 
cutting  can  have  no  material  effect  on  the  inherent  strength  and 
other  mechanical  properties  of  wood  except  in  the  outermost 
annual  ring  of  growth. 

The  popular  belief  that  sap  is  up  in  the  spring  and  summer 
and  is  down  in  the  winter  has  not  been  substantiated  by  experi- 
ment. There  are  seasonal  differences  in  the  composition  of  sap, 
but  so  far  as  the  amount  of  sap  in  a  tree  is  concerned  there  is  fully 
as  much,  if  not  more,  during  the  winter  than  in  summer.  Winter- 
cut  wood  is  not  drier,  to  begin  with,  than  summer-felled — in 
reality,  it  is  likely  to  be  wetter.* 

The  important  consideration  in  regard  to  this  question  is  the 
series  of  circumstances  attending  the  handling  of  the  timber  after 
it  is  felled.  Wood  dries  more  rapidly  in  summer  than  in  winter, 
not  because  there  is  less  moisture  at  one  time  than  another,  but 
because  of  the  higher  temperature  in  summer.  This  greater  heat 
is  often  accompanied  by  low  humidity,  and  conditions  are  favor- 
able for  the  rapid  removal  of  moisture  from  the  exposed  portions 
of  wood.  Wood  dries  by  evaporation,  and  other  things  being 
equal,  this  will  proceed  much  faster  in  hot  weather  than  in  cold. 

It  is  a  matter  of  common  observation  that  when  wood  dries 
it  shrinks,  and  if  shrinkage  is  not  uniform  in  all  directions  the 
material  pulls  apart,  causing  season  checks.  (See  Fig.  27,  page  81.) 
If  evaporation  proceeds  more  rapidly  on  the  outside  than  inside, 
the  greater  shrinkage  of  the  outer  portions  is  bound  to  result  in 
many  checks,  the  number  and  size  increasing  with  the  degree  of 
inequality  of  drying. 

In  cold  weather,  drying  proceeds  slowly  but  uniformly,  thus 
allowing  the  wood  elements  to  adjust  themselves  with  the  least 
amount  of  rupturing.  In  summer,  drying  proceeds  rapidly  and 


*  See  Record,  S.  J. :  Sap  in  relation  to  the  properties  of  wood.  Proc.  Am. 
Wood  Preservers'  Assn.,  Baltimore,  Md.,  1913,  pp.  160-166. 

Kempfer,  Wm.  H.:  The  air-seasoning  of  timber.  In  Bui.  161,  Am.  Ry. 
Eng.  Assn.,  1913,  p.  214. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        75 

irregularly,  so  that  material  seasoned  at  that  time  is  more  likely 
to  split  and  check. 

There  is  less  danger  of  sap  rot  when  trees  are  felled  in  winter 
because  the  fungus  does  not  grow  in  the  very  cold  weather,  and 
the  lumber  has  a  chance  to  season  to  below  the  danger  point  before 
the  fungus  gets  a  chance  to  attack  it.  If  the  logs  in  each  case 
could  be  cut  into  lumber  immediately  after  felling  and  given 
exactly  the  same  treatment,  for  example,  kiln-dried,  no  difference 
due  to  the  season  of  cutting  would  be  noted. 

WATER    CONTENT* 

Water  occurs  in  living  wood  in  three  conditions,  namely:  (1)  in 
the  cell  walls,  (2)  in  the  protoplasmic  contents  of  the  cells,  and 
(3)  as  free  water  in  the  cell  cavities  and  spaces.  In  heartwood  it 
occurs  only  in  the  first  and  last  forms.  Wood  that  is  thoroughly 
air-dried  retains  from  8  to  16  per  cent  of  water  in  the  cell  walls, 
and  none,  or  practically  none,  in  the  other  forms.  Even  oven- 
dried  wood  retains  a  small  percentage  of  moisture,  but  for  all 
except  chemical  purposes,  may  be  considered  absolutely  dry. 

The  general  effect  of  the  water  content  upon  the  wood  sub- 
stance is  to  render  it  softer  and  more  pliable.  A  similar  effect  of 
common  observation  is  in  the  softening  action  of  water  on  raw- 
hide, paper,  or  cloth.  Within  certain  limits  the  greater  the  water 
content  the  greater  its  softening  effect. 

Drying  produces  a  decided  increase  in  the  strength  of  wood, 
particularly  in  small  specimens.  An  extreme  example  is  the  case 
of  a  completely  dry  spruce  block  two  inches  in  section,  which 
will  sustain  a  permanent  load  four  times  as  great  as  that  which  a 
green  block  of  the  same  size  will  support. 

The  greatest  increase  due  to  drying  is  in  the  ultimate  crushing 
strength,  and  strength  at  elastic  limit  in  endwise  compression; 
these  are  followed  by  the  modulus  of  rupture,  and  stress  at  elastic 
limit  in  cross-bending,  while  the  modulus  of  elasticity  is  least 
affected.  These  ratios  are  shown  in  Table  XV,  but  it  is  to  be  noted 


*  See  Tiemann,  H.  D. :  Effect  of  moisture  upon  the  strength  and  stiffness 
of  wood.  Bui.  70,  U.  S.  Forest  Service,  Washington,  D.  C.,  1906;  also  Cir. 
108,  1907. 


76 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


that  they  apply  only  to  wood  in  a  much  drier  condition  than  is  used 
in  practice.  For  air-dry  wood  the  ratios  are  considerably  lower, 
particularly  in  the  case  of  the  ultimate  strength  and  the  elastic 
limit.  Stiffness  (within  the  elastic  limit) ,  while  following  a  similar 
law,  is  less  affected.  In  the  case  of  shear  parallel  to  the  grain,  the 
general  effect  of  drying  is  to  increase  the  strength,  but  this  is 
often  offset  by  small  splits  and  checks  caused  by  shrinkage. 

TABLE  XV 

EFFECT    OF    DRYING    ON    THE    MECHANICAL    PROPERTIES    OF    WOOD,    SHOWN    IN 

RATIO     OF    INCREASE     DUE    TO     REDUCING    MOISTURE     CONTENT    FROM 

THE    GREEN    CONDITION    TO    KILN-DRY    (3.5    PER    CENT) 

(Forest  Service  Bui.  70,  p.  89) 


KIND   OF   STRENGTH 

Longleaf 
pine 

Spruce 

Chestnut 

Crushing  strength  parallel 
to  grain 

(1)           (2) 
2.89       2.60 
2.60       2.34 
2.50       2.20 
2.90       2.55 

(1) 

3.71 

3.80 
2.81 
2.90 
2.58 
2.03 

2.26 
1.43 

(2) 
3.41 
3.49 
2.50 

2.58 
2.48 
1.95 

2.08 
1.23 

(1) 

2.83 
2.40 
2.09 
2.30 

1.55 

1.43 
1.44 

(2) 
2.55 
2.26 

1.82 
2.00 

1.47 

1.29 
1.21 

Elastic  limit  in  compres- 
sion parallel  to  grain  .  .  . 
Modulus    of    rupture    in 
bending  
Stress  at  elastic  limit  in 
bending  
Crushing  strength  at  right 
angles  to  grain 

Shearing  strength  parallel 
to  grain 

2.01       1.91 

1.63       1.47 
1.59       1.35 

Modulus   of   elasticity  in 
compression  parallel  to 
grain  
Modulus  of  elasticity  in 
bending  

NOTE. — The  figures  in  the  first  column  show  the  relative  increase  in 
strength  between  a  green  specimen  and  a  kiln-dry  specimen  of  equal  size. 
The  figures  in  the  second  column  show  the  relative  increase  of  strength 
of  the  same  block  after  being  dried  from  a  green  condition  to  3 . 5  per  cent 
moisture,  correction  having  been  made  for  shrinkage.  That  is,  in  the  first 
column  the  strength  values  per  actual  unit  of  area  are  used;  in  the  second  the 
values  per  unit  of  area  of  green  wood  which  shrinks  to  smaller  size  when 
dried. 

See  also  Cir.  108,  Fig.  1,  p.  8. 

The  moisture  content  has  a  decided  bearing  also  upon  the 
manner  in  which  wood  fails.  In  compression  tests  on  very  dry 


THE  MECHANICAL  PKOPERTIES  OF  WOOD 


77 


specimens  the  entire  piece  splits  suddenly  into  pieces  before  any 
buckling  takes  place  (see  Fig.  9,  page  18),  while  with  wet  material 
the  block  gives  way  gradually,  due  to  the  buckling  or  bending  of 


10         15         20        25        30         35        40         45 
Per  Cent  of  Moisture  Based  on  Dry  Weight 


50        55 


FIG.  24. — Relation  of  the  moisture  content  to  the  various  strength  values  of 
spruce.     FSP  =  fibre-saturation  point. 

the  walls  of  the  fibres  along  one  or  more  shearing  planes.  (See 
Fig.  14,  page  21.)  In  bending  tests  on  wet  beams,  first  failure 
occurs  by  compression  on  top  of  the  beam,  gradually  extending 
downward  toward  the  neutral  axis.  Finally  the  beam  ruptures  at 


78        THE  MECHANICAL  PROPERTIES  OF  WOOD 

the  bottom.  In  the  case  of  very  dry  beams  the  failure  is  usually 
by  splitting  or  tension  on  the  under  side  (see  Fig.  17,  page  35), 
without  compression  on  the  upper,  and  is  often  sudden  and  without 
warning,  and  even  while  the  load  is  still  increasing.  The  effect 
varies  somewhat  with  different  species,  chestnut,  for  example, 
becoming  more  brittle  upon  drying  than  do  ash,  hemlock,  and 
longleaf  pine.  The  tensile  strength  of  wood  is  least  affected  by 
drying,  as  a  rule. 

In  drying  wood  no  increase  in  strength  results  until  the  free 
water  is  evaporated  and  the  cell  walls  begin  to  dry.*  This  critical 
point  has  been  called  the  fibre-saturation  point.  (See  Fig.  24.) 
Conversely,  after  the  cell  walls  are  saturated  with  water,  any 
increase  in  the  amount  of  water  absorbed  merely  fills  the  cavities 
and  intercellular  spaces,  and  has  no  effect  on  the  mechanical 
properties.  Hence,  soaking  green  wood  does  not  lessen  its  strength 
unless  the  water  is  heated,  whereupon  a  decided  weakening  results. 

The  strengthening  effects  of  drying,  while  very  marked  in  the 
case  of  small  pieces,  may  be  fully  offset  in  structural  timbers  by 
inherent  weakening  effects  due  to  the  splitting  apart  of  the  wood 
elements  as  a  result  of  irregular  shrinkage,  and  in  some  cases 
also  to  the  slitting  of  the  cell  walls  (see  Fig.  25).  Consequently 
with  large  timbers  in  commercial  use  it  is  unsafe  to  count  upon 
any  greater  strength,  even  after  seasoning,  than  that  of  the  green 
or  fresh  condition. 

In  green  wood  the  cells  are  all  intimately  joined  together  and 
are  at  their  natural  or  normal  size  when  saturated  with  water. 
The  cell  walls  may  be  considered  as  made  up  of  little  particles 
with  water  between  them.  When  wood  is  dried  the  films  of  water 
between  the  particles  become  thinner  and  thinner  until  almost 
entirely  gone.  As  a  result  the  cell  walls  grow  thinner  with  loss  of 
moisture, — in  other  words,  the  cell  shrinks. 

It  is  at  once  evident  that  if  drying  does  not  take  place  uni- 
formly throughout  an  entire  piece  of  timber,  the  shrinkage  as  a 

*  The  wood  of  Eucalyptus  globulus  (blue  gum)  appears  to  be  an  exception 
to  this  rule.  Tiemann  says :  "The  wood  of  blue  gum  begins  to  shrink  im- 
mediately from  the  green  condition,  even  at  70  to  90  per  cent  moisture  con- 
tent, instead  of  from  30  or  25  per  cent  as  in  other  species  of  hardwoods." 
Proc.  Soc.  Am.  For.,  Washington,  Vol.  VIII,  No.  3,  Oct.,  1913,  p.  313. 


Photo  by  U.  S.  Forest  Service. 

FIG.  25. — Cross  section  of  the  wood  of  western  larch  showing  fissures  in  the  thick- 
walled  cells  of  the  late  wood.     Highly  magnified. 


Photo  by  U.  S.  Forest  Service. 

FIG.  26. — Progress  of  drying  throughout  the  length  of  a  chestnut  beam,  the  black 
spots  indicating  the  presence  of  free  water  in  the  wood.  The  first  section  at  the  left 
was  cut  one-fourth  inch  from  the  end,  the  next  one-half  inch,  the  next  one  inch,  and 
all  the  others  one  inch  apart.  The  illustration  shows  case-hardening  very  clearly. 


80        THE  MECHANICAL  PROPERTIES  OF  WOOD 

whole  cannot  be  uniform.  The  process  of  drying  is  from  the  out- 
side inward,  and  if  the  loss  of  moisture  at  the  surface  is  met  by  a 
steady  capillary  current  of  water  from  the  inside,  the  shrinkage, 
so  far  as  the  degree  of  moisture  affected  it,  would  be  uniform. 
In  the  best  type  of  dry  kilns  this  condition  is  approximated  by 
first  heating  the  wood  thoroughly  in  a  moist  atmosphere  before 
allowing  drying  to  begin. 

In  air-seasoning  and  in  ordinary  dry  kilns  this  condition  too 
often  is  not  attained,  and  the  result  is  that  a  dry  shell  is  formed 
which  encloses  a  moist  interior.  (See  Fig.  26.)  Subsequent  drying 
out  of  the  inner  portion  is  rendered  more  difficult  by  this  "  case- 
hardened  "  condition.  As  the  outer  part  dries  it  is  prevented  from 
shrinking  by  the  wet  interior,  which  is  still  at  its  greatest  volume. 
This  outer  portion  must  either  check  open  or  the  fibres  become 
strained  in  tension.  If  this  outer  shell  dries  while  the  fibres  are 
thus  strained  they  become  "  set  "  in  this  condition,  and  are  no 
longer  in  tension.  Later  when  the  inner  part  dries,  it  tends  to 
shrink  away  from  the  hardened  outer  shell,  so  that  the  inner  fibres 
are  now  strained  in  tension  and  the  outer  fibres  are  in  compression. 
If  the  stress  exceeds  the  cohesion,  numerous  cracks  Open  up,  pro- 
ducing a  "  honey-combed  "  condition,  or  "  hollow-horning,"  as  it 
is  called.  If  such  a  case-hardened  stick  of  wood  be  resawed,  the 
two  halves  will  cup  from  the  internal  tension  and  external  com- 
pression, with  the  concave  surface  inward. 

For  a  given  surface  area  the  loss  of  water  from  wood  is  always 
greater  from  the  ends  than  from  the  sides,  due  to  the  fact  that  the 
vessels  and  other  water-carriers  are  cut  across,  allowing  ready 
entrance  of  drying  air  and  outlet  for  the  water  vapor.  Water 
does  not  flow  out  of  boards  and  timbers  of  its  own  accord,  but 
must  be  evaporated,  though  it  may  be  forced  out  of  very  sappy 
specimens  by  heat.  In  drying  a  log  or  pole  with  the  bark  on,  most 
of  the  water  must  be  evaporated  through  the  ends,  but  in  the  case 
of  peeled  timbers  and  sawn  boards  the  loss  is  greatest  from  the 
surface  because  the  area  exposed  is  so  much  greater. 

The  more  rapid  drying  of  the  ends  causes  local  shrinkage, 
and  were  the  material  sufficiently  plastic  the  ends  would  become 
bluntly  tapering.  The  rigidity  of  the  wood  substance  prevents 
this  and  the  fibres  are  split  apart.  Later,  as  the  remainder  of  the 


THE  MECHANICAL  PROPERTIES  OF  WOOD        81 

stick  dries  many  of  the  checks  will  come  together,  though  some 
of  the  largest  will  remain  and  even  increase  in  size  as  the  drying 
proceeds.  (See  Fig.  27.) 

A  wood  cell  shrinks  very  little  lengthwise.     A  dry  wood  cell 
is,  therefore,  practically  of  the  same  length  as  it  was  in  a  green  or 


Photo  by  U.  S.  Forest  Service. 

FIG.  27. — Excessive  season  checking. 

saturated  condition,  but  is  smaller  in  cross  section,  has  thinner 
walls,  and  a  larger  cavity.  It  is  at  once  evident  that  this  fact 
makes  shrinkage  more  irregular,  for  wherever  cells  cross  each 
other  at  a  decided  angle  they  will  tend  to  pull  apart  upon  drying. 
This  occurs  wherever  pith  rays  and  wood  fibres  meet.  A  consid- 
erable portion  of  every  wood  is  made  up  of  these  rays,  which  for  the 
most  part  have  their  cells  lying  in  a  radial  direction  instead  of 
longitudinally.  (See  Frontispiece.)  In  pine,  over  15,000  of  these 
occur  on  a  square  inch  of  a  tangential  section,  and  even  in  oak 
the  very  large  rays  which  are  readily  visible  to  the  eye  as  flakes 


82        THE  MECHANICAL  PROPERTIES  OF  WOOD 

on  quarter-sawed  material  represent  scarcely  one  per  cent  of  the 
number  which  the  microscope  reveals. 

A  pith  ray  shrinks  in  height  and  width,  that  is,  vertically  and 
tangentially  as  applied  to  the  position  in  a  standing  tree,  but 
very  little  in  length  or  radially.  The  other  elements  of  the  wood 
shrink  radially  and  tangentially,  but  almost  none  lengthwise  or 
vertically  as  applied  to  the  tree.  Here,  then,  we  find  the  shrinkage 
of  the  rays  tending  to  shorten  a  stick  of  wood,  while  the  other 
cells  resist  it,  and  the  tendency  of  a  stick  to  get  smaller  in  circum- 
ference is  resisted  by  the  endwise  reaction  or  thrust  of  the  rays. 
Only  in  a  tangential  direction,  or  around  the  stick  in  direction  of 
the  annual  rings  of  growth,  do  the  two  forces  coincide.  Another 
factor  to  the  same  end  is  that  the  denser  bands  of  late  wood  are 
continuous  in  a  tangential  direction,  while  radially  they  are  sep- 
arated by  alternate  zones  of  less  dense  early  wood.  Consequently 
the  shrinkage  along  the  rings  (tangential)  is  fully  twice  as  much 
as  toward  the  centre  (radial).  (See  Table  XIV,  page  56.)  This 
explains  why  some  cracks  open  more  and  more  as  drying  advances. 
(See  Fig.  27.) 

Although  actual  shrinkage  in  length  is  small,  nevertheless  the 
tendency  of  the  rays  to  shorten  a  stick  produces  strains  which  are 
responsible  for  some  of  the  splitting  open  of  ties,  posts,  and  sawed 
timbers  with  box  heart.  At  the  very  centre  of  a  tree  the  wood  is 
light  and  weak,  while  farther  out  it  becomes  denser  and  stronger. 
Longitudinal  shrinkage  is  accordingly  least  at  the  centre  and 
greater  toward  the  outside,  tending  to  become  greatest  in  the 
sapwood.  When  a  round  or  a  box-heart  timber  dries  fast  it  splits 
radially,  and  as  drying  continues  the  cleft  widens  partly  on  account 
of  the  greater  tangential  shrinkage  and  also  because  the  greater 
contraction  of  the  outer  fibres  warps  the  sections  apart.  If  a 
small  hardwood  stem  is  split  while  green  for  a  short  distance  at 
the  end  and  placed  where  it  can  dry  out  rapidly,  the  sections  will 
become  bow-shaped  with  the  concave  sides  out.  These  various 
facts,  taken  together,  explain  why,  for  example,  an  oak  tie,  pole,  or 
log  may  split  open  its  entire  length  if  drying  proceeds  rapidly  and 
far  enough.  Initial  stresses  in  the  living  trees  produce  a  similar  effect 
when  the  log  is  sawn  into  boards.  This  is  especially  so  in  Euca- 
lyptus globulus  and  to  a  less  extent  with  any  rapidly  grown  wood. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        83 

The  use  of  ^-shaped  thin  steel  clamps  to  prevent  large  checks 
and  splits  is  now  a  common  practice  in  this  country  with  cross- 
ties  and  poles  as  it  has  been  for  a  long  time  in  European  countries. 
These  devices  are  driven  into  the  butts  of  the  timbers  so  as  to 
cross  incipient  checks  and  prevent  their  widening.  In  place  of 
the  regular  $-hook  another  of  crimped  iron  has  been  devised. 
(See  Fig.  28.)  Thin  straps  of  iron  with  one  tapered  edge  are  run 


Photo  by  U.  S.  Forest  Service. 

FIG.  28. — Control  of  season  checking  by  the  use  of  S-irons. 

between  intermeshing  cogs  and  crimped,  after  which  the}'  may  be 
cut  off  any  length  desired.  The  time  for  driving  $-irons  of  either 
form  is  when  the  cracks  first  appear. 

The  tendency  of  logs  to  split  emphasizes  the  importance  of 
converting  them  into  planks  or  timbers  while  in  a  green  condition. 
Otherwise  the  presence  of  large  checks  may  render  much  lumber 
worthless  which  might  have  been  cut  out  in  good  condition.  The 
loss  would  not  be  so  great  if  logs  were  perfectly  straight-grained, 
but  this  is  seldom  the  case,  most  trees  growing  more  or  less  spirally 
or  irregularly.  Large  pieces  crack  more  than  smaller  ones,  quar- 
tered lumber  less  than  that  sawed  through  and  through,  thin 
pieces,  especially  veneers,  less  than  thicker  boards. 

In  order  to  prevent  cracks  at  the  ends  of  boards,  small  straps 
of  wood  may  be  nailed  on  them  or  they  may  be  painted.  This 


84        THE  MECHANICAL  PROPERTIES  OF  WOOD 

method  is  usually  considered  too  expensive,  except  in  the  case  of 
valuable  material.  Squares  used  for  shuttles,  furniture,  gun- 
stocks,  and  tool  handles  should  always  be  protected  at  the  ends. 
One  of  the  best  means  is  to  dip  them  into  melted  paraffine,  which 
seals  the  ends  and  prevents  loss  of  moisture  there.  Another 
method  is  to  glue  paper  on  the  ends.  In  some  cases  abroad  paper 
is  glued  on  to  all  the  surfaces  of  valuable  exotic  balks.  Other 
substances  sometimes  employed  for  the  purpose  of  sealing  the 
wood  are  grease,  carbolineum,  wax,  clay,  petroleum,  linseed  oil, 
tar,  and  soluble  glass.  In  place  of  solid  beams,  built-up  material 
is  often  preferable,  as  the  disastrous  results  of  season  checks  are 
thereby  largely  overcome  or  minimized. 

TEMPERATURE 

The  effect  of  temperature  on  wood  depends  very  largely  upon 
the  moisture  content  of  the  wood  and  the  surrounding  medium. 
If  absolutely  dry  wood  is  heated  in  absolutely  dry  air  the  wood 
expands.  The  extent  of  this  expanson  is  denoted  by  a  coefficient 
corresponding  to  the  increase  in  length  or  other  dimensions  for 
each  degree  rise  in  temperature  divided  by  the  original  length 
or  other  dimension  of  the  specimen.  The  coefficient  of  linear 
expansion  of  oak  has  been  found  to  be  .00000492;  radial  expansion, 
.0000544,  or  about  eleven  times  the  longitudinal.  Spruce  expands 
less  than  oak,  the  ratio  of  radial  to  longitudinal  expansion  being 
about  six  to  one.  Metals  and  glass  expand  equally  in  all  directions, 
since  they  are  homogeneous  substances,  while  wood  is  a  compli- 
cated structure.  The  coefficient  of  expansion  of  iron  is  .0000285, 
or  nearly  six  times  the  coefficient  of  linear  expansion  of  oak  and 
seven  times  that  of  spruce.* 

Under  ordinary  conditions  wood  contains  more  or  less  moisture, 
so  that  the  application  of  heat  has  a  drying  effect  which  is  accom- 
panied by  shrinkage.  This  shrinkage  completely  obscures  the 
expansion  due  to  the  heating. 

Experiments  made  at  the  Yale  Forest  School  revealed  the  effect 
of  temperature  on  the  crushing  strength  of  wet  wood.  In  the  case 

*  See  Schlich's  Manual  of  Forestry,  Vol.  V.  (rev.  ed.),  p.  75. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


85 


of  wet  chestnut  wood  the  strength  decreases  0.42  per  cent  for  each 
degree  the  water  is  heated  above  60°  F.;  in  the  case  of  spruce  the 
decrease  is  0.32  per  cent. 

TABLE  XVI 

EFFECT   OF    STEAMING    ON   THE    STRENGTH    OF    GREEN   LOBLOLLY    PINE 

(Forest  Service,  Cir.  39) 


Cylinder  conditions 

Strength 

Steaming 

Static                 Impact 

Treatment 

Bending     Average 

Bend- 

r-,^               height         of  the 

ing         ^"^.l0"    of  drop         three 

Per- 
iod 

Pres- 
sure 

Tempera- 
ture 

modulus 
of  rup- 

allel to       causinS     strengths 
com- 

ture 

gram           plete 

failure 

Hrs. 

Lbs.  per 

°F. 

Per 

Per  cent  I  Per  cent      Per  cent 

sq.  in. 

cent 

Untreated  wood  =  100% 

4 

*230 

91.3 

79.1 

96.4       88.9 

4 

10 

238 

78.2 

93.7 

93.3       88.4 

Steam,  at  various 

4 
4 

20 

30 

253 

269 

83.3 

80.4 

84.2 
78.4 

91.4  !    86.3 

89.8       82.9 

pressures  

4 

40 

283 

78.1 

74.4 

74.0  ;    75.5 

4 

50 

292 

75.8 

71.5 

63.9  '    70.4 

4 

100 

337 

41.4 

65.0 

55.2       53.9 

1 

20 

257 

100.6 

98.6 

86.7       95.3 

2 

20 

267 

88.4 

93.0 

107.0       96.1 

3 

20 

260 

90.0 

93.6 

84.1       89.2 

Steam,  for  various   1 

4 

20 

253 

83.3 

84.2 

91.4       86.3 

periods  | 

5 

20 

253 

85.0       78.1 

84.2   .    82.4 

6 

20 

242 

95.2 

89.8 

76.0       87.0 

10 

20 

255 

73.7 

82.0 

76.0       77.2 

I 

20 

20 

258 

67.5 

65.0 

99.0       77.2 

*  It  will  be  noted  that  the  temperature  was  230°.  This  is  the  maximum 
temperature  by  the  maximum -temperature  recording  thermometer,  and  is 
due  to  the  handling  of  the  exhaust  valve.  The  average  temperature  was 
that  of  exhaust  steam. 

The  effects  of  high  temperature  on  wet  wood  are  very  marked. 
Boiling  produces  a  condition  of  great  pliability,  especially  in  the 
case  of  hardwoods.  If  wood  in  this  condition  is  bent  and  allowed 
to  dry,  it  rigidly  retains  the  shape  of  the  bend,  though  its  strength 
may  be  somewhat  reduced.  Except  in  the  case  of  very  dry  wood 


86        THE  MECHANICAL  PROPERTIES  OF  WOOD 

the  effect  of  cold  is  to  increase  the  strength  and  stiffness  of  wood. 
The  freezing  of  any  free  water  in  the  pores  of  the  wood  will  aug- 
ment these  conditions. 

The  effect  of  steaming  upon  the  strength  of  cross-ties  was  in- 
vestigated by  the  U.  S.  Forest  Service  in  1904.  The  conclusions 
were  summarized  as  follows: 

"  (1)  The  steam  at  pressure  up  to  40  pounds  applied  for 
4  hours,  or  at  a  pressure  of  20  pounds  up  to  20  hours,  increases  the 
weight  of  ties.  At  40  pounds'  pressure  applied  for  4  hours  and  at 
20  pounds  for  5  hours  the  wood  began  to  be  scorched. 

"  (2)  The  steamed  and  saturated  wood,  when  tested  imme- 
diately after  treatment,  exhibited  weaknesses  in  proportion  to  the 
pressure  and  duration  of  steaming.  (See  Table  XVI.)  If  allowed 
to  air-dry  subsequently  the  specimens  regained  the  greater  part 
of  their  strength,  provided  the  pressure  and  duration  had  not 
exceeded  those  cited  under  (1).  Subsequent  immersion  in  water 
of  the  steamed  wood  and  dried  specimens  showed  that  they  were 
weaker  than  natural  wood  similarly  dried  and  resoaked."  * 

"  (3)  A  high  degree  of  steaming  is  injurious  to  wood  in  strength 
and  spike-holding  power.  The  degree  of  steaming  at  which  pro- 
nounced harm  results  will  depend  upon  the  quality  of  the  wood 
and  its  degree  of  seasoning,  and  upon  the  pressure  (temperature) 
of  steam  and  the  duration  of  its  application.  For  loblolly  pine 
the  limit  of  safety  is  certainly  30  pounds  for  4  hours,  or  20  pounds 
for  6  hours."! 

Experiments  made  at  the  Yale  Forest  School  showed  that 
steaming  above  30  pounds'  gauge  pressure  reduces  the  strength  of 
wood  permanently  while  wet  from  25  to  75  per  cent. 

PRESERVATIVES 

The  exact  effects  of  chemical  impregnation  upon  the  mechanical 
properties  of  wood  have  not  been  fully  determined,  though  they 
have  been  the  subject  of  considerable  investigation.^  More 

*  Cir.  39.     Experiments  on  the  strength  of  treated  timber,  p.  18. 
t  Ibid.,  p.  21.     See  also  Cir.  108,  p.  19,  table  5. 

{Hatt,  W.  K.:  Experiments  on  the  strength  of  treated  timber.  Cir.  39, 
U.  S.  Forest  Service,  1906,  p.  31. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        87 

depends  upon  the  method  of  treatment  than  upon  the  preserva- 
tives used.  Thus  preliminary  steaming  at  too  high  pressure  or 
for  too  long  a  period  will  materially  weaken  the  wood.  (See 
Temperature,  supra.) 

The  presence  of  zinc  chloride  does  not  weaken  wood  under 
static  loading,  although  the  indications  are  that  the  wood  becomes 
brittle  under  impact.  If  the  solution  is  too  strong  it  will  decom- 
pose the  wood. 

Soaking  in  creosote  oil  causes  wood  to  swell,  and  accordingly 
decreases  the  strength  to  some  extent,  but  not  nearly  so  much 
so  as  soaking  in  water.* 

Soaking  in  kerosene  seems  to  have  no  significant  weakening 
effect.t 

*  Teesdale,  Clyde  H.:  The  absorption  of  creosote  by  the  cell  walls  of 
wood.  Cir.  200,  U.  S.  Forest  Service,  1912,  p.  7. 

f  Tiemann,  H.  D. :  Effect  of  moisture  upon  the  strength  and  stiffness  of 
wood  Bui.  70,  U.  S.  Forest  Service,  1907,  pp.  122-123,  tables  43-44. 


PART  III 
TIMBER  TESTING* 

WORKING    PLAN 

PRELIMINARY  to  making  a  series  of  timber  tests  it  is  very 
important  that  a  working  plan  be  prepared  as  a  guide  to  the 
investigation.  This  should  embrace :  (1)  the  purpose  of  the  tests; 
(2)  kind,  size,  condition,  and  amount  of  material  needed ;  (3)  full 
description  of  the  system  of  marking  the  pieces ;  (4)  details  of  any 
special  apparatus  and  methods  employed;  (5)  proposed  method 
of  analyzing  the  data  obtained  and  the  nature  of  the  final  report. 
Great  care  should  be  taken  in  the  preparation  of  this  plan  in 
order  that  all  problems  arising  may  be  anticipated  so  far  as 
possible  and  delays  and  unnecessary  work  avoided.  A  compre- 
hensive study  of  previous  investigations  along  the  same  or  related 
lines  should  prove  very  helpful  in  outlining  the  work  and  prepar- 
ing the  report.  (For  sample  working  plan  see  Appendix,  page  127.) 

FORMS    OF    MATERIAL   TESTED 

In  general,  four  forms  of  material  are  tested,  namely:  (1)  large 
timbers,  such  as  bridge  stringers,  car  sills,  large  beams,  and 
other  pieces  five  feet  or  more  in  length,  of  actual  sizes  and  grades 
in  common  use;  (2)  built-up  structural  forms  and  fastenings,  such 
as  built-up  beams,  trusses,  and  various  kind  of  joints;  (3)  small 
clear  pieces,  such  as  are  used  in  compression,  shear,  cleavage,  and 
small  cross-breaking  tests;  (4)  manufactured  articles,  such  as  axles, 
spokes,  shafts,  wagon-tongues,  cross-arms,  insulator  pins,  barrels, 
and  packing  boxes. 

As  the  moisture  content  is  of  fundamental  importance  (see 
Water  Content,  pages  75-84),  all  standard  tests  are  usually 

*  The  methods  of  timber  testing  described  here  are  for  the  most  part  those 
employed  by  the  U.  S.  Forest  Service.  See  Cir.  38  (rev.  ed.),  1909. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        89 

made  in  the  green  condition.  Another  series  is  also  usually  run 
in  an  air-dry  condition  of  about  12  per  cent  moisture.  In  all 
cases  the  moisture  is  very  carefully  determined  and  stated  with 
the  results  in  the  tables. 

SIZE    OF    TEST    SPECIMENS 

The  size  of  the  test  specimen  must  be  governed  largely  by  the 
purpose  for  which  the  test  is  made.  If  the  effect  of  a  single  factor, 
such  as  moisture,  is  the  object  of  experiment,  it  is  necessary  to  use 
small  pieces  of  wood  in  order  to  eliminate  so  far  as  possible  all 
disturbing  factors.  If  the  specimens  are  too  large,  it  is  impossible 
to  secure  enough  perfect  pieces  from  one  tree  to  form  a  series  for 
various  tests.  Moreover,  the  drying  process  with  large  timbers  is 
very  difficult  and  irregular,  and  requires  a  long  period  of  time, 
besides  causing  checks  and  internal  stresses  which  may  obscure  the 
results  obtained. 

On  the  other  hand,  the  smaller  the  dimensions  of  the  test 
specimen  the  greater  becomes  the  relative  effect  of  the  inherent 
factors  affecting  the  mechanical  properties.  For  example,  the 
effect  of  a  knot  of  given  size  is  more  serious  in  a  small  stick  than  in 
a  large  one.  Moreover,  the  smaller  the  specimen  the  fewer  growth 
rings  it  contains,  hence  there  is  greater  opportunity  for  variation 
due  to  irregularities  of  grain. 

Tests  on  large  timbers  are  considered  necessary  to  furnish 
designers  data  on  the  probable  strength  of  the  different  sizes 
and  grades  of  timber  on  the  market;  their  coefficients  of  elasticity 
under  bending  (since  the  stiffness  rather  than  the  strength  often 
determines  the  size  of  a  beam) ;  and  the  manner  of  failure,  whether 
in  bending  fibre  stress  or  horizontal  shear.  It  is  believed  that 
this  information  can  only  be  obtained  by  direct  tests  on  the 
different  grades  of  car  sills,  stringers,  and  other  material  in  com- 
mon use. 

When  small  pieces  are  selected  for  test  they  very  often  are  clear 
and  straight-grained,  and  thus  of  so  much  better  grade  than  the 
large  sticks  that  tests  upon  them  may  not  yield  unit  values 
applicable  to  the  larger  sizes.  Extensive  experiments  show,  how- 
ever, (1)  that  the  modulus  of  elasticity  is  approximately  the 


90        THE  MECHANICAL  PROPERTIES  OF  WOOD 

same  for  large  timbers  as  for  small  clear  specimens  cut  from  them, 
and  (2)  that  the  fibre  stress  at  elastic  limit  for  large  beams  is, 
except  in  the  weakest  timbers,  practically  equal  to  the  crushing 
strength  of  small  clear  pieces  of  the  same  material.* 

MOISTURE    DETERMINATION 

In  order  for  tests  to  be  comparable,  it  is  necessary  to  know  the 
moisture  content  of  the  specimens  at  the  zone  of  failure.  This  is 
determined  from  disks  an  inch  thick  cut  from  the  timber  immedi- 
ately after  testing. 

In  cases,  as  in  large  beams/  where  it  is  desirable  to  know  not 
only  the  average  moisture  content  but  also  its  distribution  through 
the  timber,  the  disks  are  cut  up  so  as  to  obtain  an  outside,  a 
middle,  and  an  inner  portion,  of  approximately  equal  areas.  Thus 
in  a  section  10"  x  12"  the  outer  strip  would  be  one  inch  wide,  and 
the  second  one  a  little  more  than  an  inch  and  a  quarter.  Moisture 
determinations  are  made  for  each  of  the  three  portions  separately. 

The  procedure  is  as  follows : 

(1)  Immediately  after  sawing,  loose  splinters  are  removed  and 
each  section  is  weighed. 

(2)  The  material  is  put  into  a  drying  oven  at  100°  C.  (212°  F.) 
and  dried  until  the  variation  in  weight  for  a  period  of  twenty-four 
hours  is  less  than  0.5  per  cent. 

(3)  The  disk  is  again  carefully  weighed. 

(4)  The  loss  in  weight  expressed  in  per  cent  of  the  dry  weight 
indicates  the  moisture  content  of  the  specimen  from  which  the 
specimen  was  cut. 

MACHINE    FOR   STATIC    TESTS 

The  standard  screw  machines  used  for  metal  tests  are  also 
used  for  wood,  but  in  the  case  of  wood  tests  the  readings  must  be 
taken  "  on  the  fly,"  and  the  machine  operated  at  a  uniform  speed 
without  interruption  from  beginning  to  end  of  the  test.  This  is 
on  account  of  the  time  factor  in  the  strength  of  wood.  (See  Speed 
of  Testing  Machine,  page  92.) 


*  Bui.  108,  U.  S.  Forest  Service:  Tests  of  structural  timbers,  pp.  53-54. 


THE  MECHANICAL  PROPERTIES  OF  WOOD        91 

The  standard  machines  for  static  tests  can  be  used  for  trans- 
verse bending,  compression,  tension,  shear,  and  cleavage.  A 
common  form  consists  of  three  main  parts,  namely:  (1)  the 
straining  mechanism,  (2)  the  weighing  apparatus,  and  (3)  the 
machinery  for  communicating  motion  to  the  screws. 

The  straining  mechanism  consists  of  two  parts,  one  of  which 
is  a  movable  crosshead  operated  by  four  (sometimes  two  or  three) 
upright  steel  straining  screws  which  pass  through  openings  in  the 
platform  and  bear  upward  on  the  bed  of  the  machine  upon  which 
the  weighing  platform  rests  as  a  fulcrum.  At  the  lower  ends  of 
these  screws  are  geared  nuts  all  rotated  simultaneously  by  a 
system  of  gears  which  cause  the  movable  crosshead  to  rise  and 
fall  as  desired 

The  stationary  part  of  the  straining  mechanism,  which  is  used 
only  for  tension  and  cleavage  tests,  consists  of  a  steel  cage  above 
the  movable  crosshead  and  rests  directly  upon  the  weighing 
platform.  The  top  of  the  cage  contains  a  square  hole  into  which 
one  end  of  the  test  specimen  may  be  clamped,  the  crosshead  con- 
taining a  similar  clamp  for  the  other  end,  in  making  tension  tests. 

For  testing  long  beams  a  special  form  of  machine  with  an 
extended  platform  is  used.  (See  Fig.  29,  page  95.) 

The  weighing  platform  rests  upon  knife  edges  carried  by 
primary  levers  of  the  weighing  apparatus,  the  fulcrum  being  on 
the  bed  of  the  machine,  and  any  pressure  upon  it  is  directly  trans- 
mitted through  a  series  of  levers  to  the  weighing  beam.  This  beam 
is  adjusted  by  means  of  a  poise  running  on  a  screw.  In  opera- 
tion the  beam  is  kept  floating  by  means  of  another  poise  moved 
back  and  forth  by  a  screw  which  is  operated  by  a  hand  wheel  or 
automatically.  The  larger  units  of  stress  are  read  from  the 
graduations  along  the  side  of  the  beam,  while  the  intermediate 
smaller  weights  are  observed  on  the  dial  on  the  rear  end  of  the 
beam. 

The  machine  is  driven  by  power  from  a  shaft  or  a  motor  and 
is  so  geared  that  various  speeds  are  obtainable.  One  man  can 
operate  it. 

In  making  tests  the  operation  of  the  straining  screws  is  always 
downward  so  as  to  bring  pressure  to  bear  upon  the  weighing 
platform.  For  tests  in  tension  and  cleavage  the  specimen  is  placed 


92        THE  MECHANICAL  PROPERTIES  OF  WOOD 

between  the  top  of  the  stationary  cage  and  the  movable  head 
and  subjected  to  a  pull.  For  tests  in  transverse  bending,  compres- 
sion, and  cleavage  the  specimen  is  placed  between  the  movable 
head  and  the  platform,  and  a  direct  compression  force  applied. 

Testing  machines  are  usually  calibrated  to  a  portion  of  their 
capacity  before  leaving  the  factory.  The  delicacy  of  the  weighing 
levers  is  verified  by  determining  the  number  of  pounds  necessary 
to  move  the  beam  between  the  stops  while  a  load  of  1,000  pounds 
rests  on  the  platform.  The  usual  requirement  is  that  ten  pounds 
should  accomplish  this  movement. 

The  size  of  machine  suitable  for  compression  tests  on  2"  X  2" 
sticks  or  for  2"X  2"  beams  with  26  to  36-inch  span  has  a  capacity 
of  30,000  pounds. 

SPEED    OF    TESTING    MACHINE 

In  instructions  for  making  static  tests  the  rate  of  application 
of  the  stress,  i.e.,  the  speed  of  the  machine,  is  given  because  the 
strength  of  wood  varies  with  the  speed  at  which  the  fibres  are 
strained.  The  speed  of  the  crosshead  of  the  testing  machine  is 
practically  never  constant,  due  to  mechanical  defects  of  the 
apparatus  and  variations  in  the  speed  of  the  motor,  but  so  long  as 
it  does  not  exceed  25  per  cent  the  results  will  not  be  appreciably 
affected.  In  fact,  a  change  in  speed  of  50  per  cent  will  not  cause 
the  strength  of  the  wood  to  vary  more  than  2  per  cent.* 

Following  are  the  formulae  used  in  determining  the  speed  of 
the  movable  head  of  the  machine  in  inches  per  minute  (ri)  : 
(1)    For  endwise  compression    .         .         .       n  =  Zl 


(2)    For  beams  (centre  loading) 


-~-r 


ZP 
(3)    For  beams  (third-point  loading)    .       .       n  =  ^-TT 

Z  =  rate  of  fibre  strain  per  inch  of  fibre  length. 
/  =  span  of  beam  or  length  of  compression  speci- 

men. 
h  =  height  of  beam. 

*  See  Tiemann,  Harry  Donald:  The  effect  of  the  speed  of  testing  upon  the 
strength  and  the  standardization  of  tests  for  speed.  Proc.  Am.  Soc.  for  Test- 
ing Materials,  Vol.  VIII,  Philadelphia,  1908. 


THE  MECHANICAL  PKOPERTIES  OF  WOOD 


93 


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NOTE.—  The  usual  speeds  of  testing  at  the  U.  S.  Forest  Service  laboratory  are  at  rates  of  fibre  strain  of  15  and  10  ten-thousandths  in.  per  min.  per  in.  for  compression  and 
bending  respectively. 

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94        THE  MECHANICAL  PROPERTIES  OF  WOOD 

The  values  commonly  used  for  Z  are  as  follows  : 
Bending  large  beams  Z  =  0.0007 

Bending  small  beams  .  .  .  .  Z  =  0.0015 
Endwise  compression  —  large  specimens  Z  =  0.0015 
Endwise  compression  —  small  Z  =  0.003 

Right-angled  compression  —  large   "      Z  =  0.007 
Right-angled  compression  —  small  "      Z  =  0.015 
Shearing  parallel  to  the  grain    .       .       Z  =  0.015 
Example:  At  what  speed  should  the  crosshead  move  to  give  the 
required  rate  of  fibre  strain  in  testing  a  small  beam  2"  X  2"  X  30". 
(Span  =  28".)     Substituting  these  values  in  equation  (2)  above: 

0.0015X282 
n  =  --  £—-  ~  --  =  0.1  inch  per  minute. 


In  order  that  tests  may  be  intelligently  compared,  it  is  im- 
portant that  account  be  taken  of  the  speed  at  which  the  stress 
was  applied.  In  determining  the  basis  for  a  ratio  between  time 
and  strength  the  rate  of  strain,  which  is  controllable,  and  not  the 
ratio  of  stress,  which  is  circumstantial,  should  be  used.  In  other 
words,  the  rate  at  which  the  movable  head  of  the  testing  machine 
descends  and  not  the  rate  of  increase  in  the  load  is  to  be  regulated. 
This  ratio,  to  which  the  name  speed-strength  modulus  has  been 
given,  may  be  expressed  as  a  coefficient  which,  if  multiplied  into 
any  proportional  change  in  speed,  will  give  the  proportional 
change  in  strength.  This  ratio  is  derived  from  empirical  curves. 
(See  Table  XVII.) 

BENDING    LARGE    BEAMS 

Apparatus:  Astatic  bending  machine  (described  above)  ,  with 
a  special  crosshead  for  third-point  loading  and  a  long  platform 
bearing  knife-edge  supports,  is  required.  (See  Fig.  29.) 

Preparing  the  material:  Standard  sizes  and  grades  of  beams 
and  timbers  in  common  use  are  employed.  The  ends  are  roughly 
squared  and  the  specimen  weighed  and  measured,  taking  the 
cross-sectional  dimensions  midway  of  the  length.  Weights  should 
be  to  the  nearest  pound,  lengths  to  the  nearest  0.1  inch,  and 
cross-sectional  dimensions  to  the  nearest  0.01  inch. 

Marking  and  sketching:  The  butt  end  of  the  beam  is  marked 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


95 


A  and  the  top  end  B.  While  facing  A,  the  top  side  is  marked  a, 
the  right  hand  6,  the  bottom  c,  the  left  hand  d.  Sketches  are  made 
of  each  side  and  end,  showing  (1)  size,  location,  and  condition  of 
knots,  checks,  splits,  and  other  defects;  (2)  irregularities  of  grain; 
(3)  distribution  of  heartwood  and  sapwood;  and  on  the  ends:  (4) 
the  location  of  the  pith  and  the  arrangement  of  the  growth  rings, 
(5)  number  of  rings  per  inch,  and  (6)  the  proportion  of  late  wood. 


Photo  by  U.  S.  Forest  Service. 

FIG.  29. — Static    bending    test    on    large    beam.     Note    arrangement  of  wire    and 
scale  for  measuring  deflection;  also  method  of  applying  load  at  "third-points." 


The  number  of  rings  per  inch  and  the  proportion  of  late  wood 
should  always  be  determined  along  a  radius  or  a  line  normal  to 
the  rings.  The  average  number  of  rings  per  inch  is  the  total 
number  of  rings  divided  by  the  length  of  the  line  crossing  them. 
The  proportion  of  late  wood  is  equal  to  the  sum  of  the  widths  of 
the  late  wood  crossed  by  the  line,  divided  by  the  length  of  the 
line.  Rings  per  inch  should  be  to  the  nearest  0.1;  late  wood  to 
the  nearest  0.1  per  cent. 

Since  in  large  beams  a  great  variation  in  rate  of  growth  and 


96        THE  MECHANICAL  PROPERTIES  OF  WOOD 

relative  amount  of  late  wood  is  likely  in  different  parts  of  the 
section,  it  is  advisable  to  consider  the  cross  section  in  three 
volumes,  namely,  the  upper  and  lower  quarters  and  the  middle 
half.  The  determination  should  be  made  upon  each  volume  sep- 
arately, and  the  average  for  the  entire  cross  section  obtained 
from  these  results. 

At  the  conclusion  of  the  test  the  failure,  as  it  appears  on  each 
surface,  is  traced  on  the  sketches,  with  the  failures  numbered  in 
the  order  of  their  occurrence.  If  the  beam  is  subsequently  cut 
up  and  used  for  other  tests  an  additional  sketch  may  be  desirable 
to  show  the  location  of  each  piece. 

Adjusting  specimen  in  machine:  The  beam  is  placed  in  the 
machine  with  the  side  marked  a  on  top,  and  with  the  ends  pro- 
jecting equally  beyond  the  supports.  In  order  to  prevent  crushing 
of  the  fibre  at  the  points  where  the  stress  is  applied  it  is  necessary 
to  use  bearing  blocks  of  maple  or  other  hard  wood  with  a  convex 
surface  in  contact  with  the  beam.  Roller  bearings  should  be 
placed  between  the  bearing  blocks  and  the  knife  edges  of  the 
crosshead  to  allow  for  the  shortening  due  to  flexure.  (See  Fig.  29.) 
Third-point  loading  is  used,  that  is,  the  load  is  applied  at  two  points 
one-third  the  span  of  the  beam  apart.  (See  Fig.  30.)  This  affords 
a  uniform  bending  moment  throughout  the  central  third  of  the 
beam. 

Measuring  the  deflection:  The  method  of  measuring  the  deflec- 
tion should  be  such  that  any  compression  at  the  points  of  support 
or  at  the  application  of  the  load  will  not  affect  the  reading.  This 
may  be  accomplished  by  driving  a  small  nail  near  each  end  of 
the  beam,  the  exact  location  being  on  the  neutral  plane  and  verti- 
cally above  each  knife-edge  support.  Between  these  nails  a  fine 
wire  is  stretched  free  of  the  beam  and  kept  taut  by  means  of  a 
rubber  band  or  coiled  spring  on  one  end.  Behind  the  wire  at  a 
point  on  the  beam  midway  between  the  supports  a  steel  scale 
graduated  to  hundredths  of  an  inch  is  fastened  vertically  by 
means  of  thumb-tacks  or  small  screws  passing  through  holes  in  it. 
Attachment  should  be  made  on  the  neutral  plane. 

The  first  reading  is  made  when  the  scale  beam  is  balanced  at 
zero  load,  and  afterward  at  regular  increments  of  the  load  which 
is  applied  continuously  and  at  a  uniform  speed.  (See  Speed  of 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


97 


Testing  Machine,  page  92.)  If  desired,  however,  the  load  may 
be  read  at  regular  increments  of  deflection.  The  deflection  readings 
should  be  to  the  nearest  0.01  inch.  To  avoid  error  due  to  parallax, 
the  readings  may  be  taken  by  means  of  a  reading  telescope  about 
ten  feet  distant  and  approximately  on  a  level  with  the  wire.  A 
mirror  fastened  to  the  scale  will  increase  the  accuracy  of  the  read- 
ings if  the  telescope  is  not  used.  As  in  all  tests  on  timber,  the 


<                 M  7                    — 

«--                      1/7 

\ 

*=•                       >3  i                         - 

*  i  _  _ 

J 


FIG.  30. — Two  methods  of  loading  a  beam,  namely,  third-point  loading  (upper), 
and  centre  loading  (lower). 

strain  must  be  continuous  to  rupture,  not  intermittent,  and 
readings  must  be  taken  "  on  the  fly."  The  weighing  beam  is  kept 
balanced  after  the  yield  point  is  reached  and  the  maximum  load, 
and  at  least  one  point  beyond  it,  noted. 

Log  of  the  test:  The  proper  log  sheet  for  this  test  consists  of 
a  piece  of  cross-section  paper  with  space  at  the  margin  for  notes. 
(See  Fig.  32,  page  101.)  The  load  in  some  convenient  unit  (1,000 
to  10,000  pounds,  depending  upon  the  dimensions  of  the  specimen) 
is  entered  on  the  ordinates,  the  deflection  in  tenths  of  an  inch  on 
the  abscissae.  The  increments  of  load  should  be  chosen  so  as  to 


98  THE    ME€HANICAL    PROPERTIES    OF   WOOD 

furnish  about  ten  points  on  the  stress-strain  diagram  below  the 
elastic  limit. 

As  the  readings  of  the  wire  on  the  scale  are  made  they  are  en- 
tered directly  in  their  proper  place  on  the  cross-section  paper. 
In  many  cases  a  test  should  be  continued  until  complete  failure 
results.  The  points  where  the  various  failures  occur  are  indicated 
on  the  stress-strain  diagram.  A  brief  description  of  the  failure 
is  made  on  the  margin  of  the  log  sheet,  and  the  form  traced  on  the 
sketches. 

Disposal  of  the  specimen:  Two  one-inch  sections  are  cut  from 
the  region  of  failure  to  be  used  in  determining  the  moisture  con- 
tent. (See  Moisture  Determination,  page  90.)  A  two-inch 
section  may  be  cut  for  subsequent  reference  and  identification, 
and  possible  microscopic  study.  The  remainder  of  the  beam  may 
be  cut  into  small  beams  and  compression  pieces. 

Calculating  the  results:  The  formulae  used  in  calculating  the 
results  of  tests  on  large  rectangular  simple  beams  loaded  at  third 
points  of  the  span  are  as  follows : 

m  •  7  =  °'75P  (2}   R  -  ;(P  +  (X75Ty) 

(1)   J  '       b  h  b  V 

I  (P,  +  0.75  W)  P,  I" 

<2>   r  =  ~    Tie-  (4)  E  -  4TI5TT3 

0.87  P,  D 
~^V~ 

b,  h,  I  =  breadth,  height,  and  span  of  specimen,  inches. 
D  =  total  deflection  at  elastic  limit,  inches. 
P  =  maximum  load,  pounds. 
Pi  =  load  at  elastic  limit,  pounds. 
E  =  modulus  of  elasticity,  pounds  per  square  inch. 
r  =  fibre  stress  at  elastic  limit,  pounds  per  sq.  inch. 
R  =  modulus  of  rupture,  pounds  per  square  inch. 
S  =  elastic  resilience  or  work  to  elastic  limit,  inch-pounds 

per  cu.  in. 
J  =  greatest  calculated  longitudinal  shear,  pounds 

per  square  inch. 

V  =  volume  of  beam,  cubic  inches. 
W  =  weight  of  the  beam. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


99 


In  large  beams  the  weight  should  be  taken  into  account  in  calcu- 
lating the  fibre  stress.  In  (2)  and  (3)  three-fourths  of  the  weight  of 
the  beam  is  added  to  the  load  for  this  reason. 


BENDING    SMALL   BEAMS 

Apparatus:  An  ordinary  static  bending  machine,  a  steel 
I-beam  bearing  two  adjustable  knife-edge  supports  to  rest  on  the 
platform,  and  a  special  deflectometer,  are  required.  (See  Fig.  31.) 

Preparing  the  material:  The  specimens  may  be  of  any  conve- 
nient size,  though  beams  2"X2"  X30"  tested  over  a  28-inch  span, 


Photo  by  U.  S.  Forest  Service. 

FIG.  31. — Static  bending  test  on  small  beam.  Note  the  use  of  the  deflectometer 
with  indicator  and  dial  for  measuring  the  deflection;  also  roller  bearings  between 
beam  and  supports. 

are  considered  best.  The  beams  are  surfaced  on  all  four  sides, 
care  being  taken  that  they  are  not  damaged  by  the  rollers  of  the 
surfacing  machine.  Material  for  these  tests  is  sometimes  cut 
from  large  beams  after  failure.  The  specimens  are  carefully 
weighed  in  grams,  and  all  dimensions  measured  to  the  nearest 
0.01  inch.  If  to  be  tested  in  a  green  or  fresh  condition  the  speci- 


100  THE    MECHANICAL   PROPERTIES   OF   WOOD 

mens  should  be  kept  in  a  damp  box  or  covered  with  moist  sawdust 
until  needed.  No  defects  should  be  allowed  in  these  specimens. 

Marking  and  sketching :  Sketches  are  made  of  each  end  of  the 
specimen  to  show  the  character  of  the  growth,  and  after  testing, 
the  manner  of  failure  is  shown  for  all  four  sides.  In  obtaining 
data  regarding  the  rate  of  growth  and  the  proportion  of  late  wood 
the  same  procedure  is  followed  as  with  large  beams. 

Adjusting  specimen  in  machine:  The  beam  should  be  correctly 
centred  in  the  machine  and  each  end  should  have  a  plate  with 
roller  bearings  between  it  and  the  support.  Centre  loading  is 
used.  Between  the  movable  head  of  the  machine  and  the  speci- 
men is  placed  a  bearing  block  of  maple  or  other  hard  wood,  the 
lower  surface  of  which  is  curved  in  a  direction  along  the  beam,  the 
curvature  of  which  should  be  slightly  less  than  that  of  the  beam 
at  rupture,  in  order  to  prevent  the  edges  from  crushing  into  the 
fibres  of  the  test  piece. 

Measuring  the  deflection:  The  method  of  measuring  deflection 
of  large  beams  can  be  used  for  small  sizes,  but  because  of  the 
shortness  of  the  span  and  consequent  slight  deformation  in  the 
latter,  it  is  hardly  accurate  enough  for  good  work.  The  special 
deflectometer  shown  in  Fig.  31  allows  closer  reading,  as  it 
magnifies  the  deflection  ten  times.  It  rests  on  two  small  nails 
driven  in  the  beam  on  the  neutral  plane  and  vertically  above 
the  supports.  The  fine  wire  on  the  wheel  at  the  base  of  the  indi- 
cator is  attached  to  another  small  nail  driven  in  the  beam  on  the 
neutral  plane  midway  between  the  end  nails.  All  three  nails  should 
be  in  place  before  the  beam  is  put  into  the  machine.  The  indicator 
is  adjustable  by  means  of  a  thumb-screw  at  the  base  and  is  set  at 
zero  before  the  load  is  applied.  Deflections  are  read  to  the  nearest 
0.001  inch.  For  rate  of  application  of  load  see  Speed  of  Testing 
Machine,  page  92.  The  speed  should  be  uniform  from  start  to 
finish  without  stopping.  Readings  must  be  made  "  on  the  fly." 

Log  of  the  test:  The  log  sheets  used  for  small  beams  (see  Fig. 
32)  are  the  same  as  for  large  sizes  and  the  procedure  is  practically 
identical.  The  stress-strain  diagram  is  continued  to  or  beyond 
the  maximum  load,  and  in  a  portion  of  the  tests  should  be  contin- 
ued to  six-inch  deflection  or  until  the  specimen  fails  to  support  a 
load  of  200  pounds.  Deflection  readings  for  equal  increments  of 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


FORM  512 
(Supersedes  Form  176) 


Pro j  ect  No.  _"_  _  1 
Working  Plan  No 


U.S.  DEPARTMENT  OF  AGRICULTURE 
FOREST  SERVICE 


101 

Timber  Test  Log 


Station      Madison 


Date       3/fO/U 


Ship.  No._  _  ~'A _  _         Stick  No._  . : 


pecies       W.  Yellow  Pine 
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Deflection  in  inches 

FIG.  32. — Sample  log  sheet,  giving  full  details  of  a  transverse  bending  test  on  a 
small  pine  beam. 


102  THE    MECHANICAL   PROPERTIES    OF   WOOD 

load  are  taken  until  well  beyond  the  elastic  limit,  after  which  the 
scale  beam  is  kept  balanced  and  the  load  read  for  each  0.1  inch 
deflection.  The  load  and  deflection  at  first  failure,  the  maximum 
load,  and  any  points  of  sudden  change  should  be  shown  on  the 
diagram,  even  though  they  do  not  occur  at  one  of  the  regular 
points.  A  brief  description  of  the  failure  and  the  nature  of  any 
defects  is  entered  on  the  log  sheet. 

Calculating  the  results:    The  formulae  used  in  calculating  the 
results  of  tests  on  small  rectangular  simple  beams  are  as  follows  : 


- 

"  2  V 

The  same  legend  is  used  as  on  page  98.    The  weight  of  the  beam 
itself  is  disregarded. 

ENDWISE    COMPRESSION 

Apparatus:  An  ordinary  static  testing  machine  and  a  com- 
pressometer  are  required.  (See  Fig.  33.) 

Preparing  the  material:  Two  classes  of  specimens  are  com- 
monly used,  namely,  (1)  posts  24  inches  in  length,  and  (2)  small 
clear  blocks  approximately  2"  X  2"  X  8".  The  specimens  are  sur- 
faced on  all  four  sides  and  both  ends  squared  smoothly  and  evenly. 
They  are  carefully  weighed,  measured,  rate  of  growth  and  pro- 
portion of  late  wood  determined,  as  in  bending  tests.  After  the 
test  a  moisture  section  is  cut  and  weighed.  Ordinarily  these  speci- 
mens should  be  free  from  defects. 

Sketching:  Sketches  are  made  of  each  end  of  the  specimens  to 
show  the  character  ,of  the  growth.  After  testing,  the  manner  of 
failure  is  \shcwi,\?  for  all  four  sides,  and  the  various  parts  of  the 
failure  are  numbered  in  the  order  of  their  occurrence. 
;  ',  ^A/jttfustir.ci'tSpectmen.-in  machine:  The  compressometer  collars 
are  adjusted,  the  distance  between  them  being  20  inches  for  the 
posts  and  6  inches  for  the  blocks.  If  the  two  ends  of  the  blocks  are 
not  exactly  parallel  a  ball-and-socket  block  can  be  placed  between 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


103 


the  upper  end  of  the  specimen  and  the  movable  head  of  the 
machine  to  overcome  the  irregularity.  If  the  blocks  are  true 
they  can  simply  be  stood  on  end  upon  the  platform  and  the 
movable  head  allowed  to  press  directly  upon  the  upper  end. 

Measuring  the  deformation:  The  deformation  is  measured  by  a 
compressometer.  (See  Fig.  33.)  The  latter  registers  to  0.001  inch. 
In  the  case  of  posts  the  compression  between  the  collars  is  com- 


Photo  by  U.  S.  Forest  Service. 

FIG.  33. — Endwise  compression  test,  showing  method  of  measuring  the  deforma- 
tion by  means  of  a  compre§someter. 

municated  to  the  four  points  on  the  arms  by  means  of  brass 
rods;  with  short  blocks,  as  in  Fig.  33,  the  points  of  the  arms 
are  in  direct  contact  with  the  collars.  The  operator  lowers  the 
fulcrum  of  the  apparatus  by  moving  the  micrometer  screws  at 
such  a  rate  that  the  set-screw  in  the  rear  end  of  the  upper  lever 
is  kept  barely  touching  the  fixed  arm  below  it,  being  guided  by 
a  bell  operated  by  electric  contact. 

Log  of  the  test:     The  load  is  applied  continuously  at  a  uniform 
rate    of    speed.       (See    Speed  of  Testing  Machine,    page    92.) 


104  THE   MECHANICAL   PROPERTIES    OF   WOOD 

Readings  are  taken  from  the  scale  of  the  compressometer  at  reg- 
ular increments  of  either  load  or  compression.  The  stress-strain 
diagram  is  continued  to  at  least  one  deformation  point  beyond 
the  maximum  load,  and  in  event  of  sudden  failure,  the  direction 
of  the  curve  beyond  the  maximum  point  is  indicated.  A  brief 
description  of  the  failure  is  entered  on  the  log  sheet.  (See  Fig.  34.) 

In  short  specimens  the  failure  usually  occurs  in  one  or  several 
planes  diagonal  to  the  axis  of  the  specimen.  If  the  ends  are  more 
moist  than  the  middle  a  crushing  may  occur  on  the  extreme 
ends  in  a  horizontal  plane.  Such  a  test  is  not  valid  and  should 
always  be  culled.  If  the  grain  is  diagonal  or  the  stress  is  unevenly 
applied  a  diagonal  shear  may  occur  from  top  to  bottom  of  the 
test  specimen.  Such  tests  are  also  invalid  and  should  be  culled. 
When  the  plane  (or  several  planes)  of  failure  occurs  through  the 
body  of  the  specimen  the  test  is  valid.  It  may  sometimes  be 
advantageous  to  allow  the  extreme  ends  to  dry  slightly  before 
testing  in  order  to  bring  the  planes  of  failure  within  the  body. 
This  is  a  perfectly  legitimate  procedure  provided  no  drying  is 
allowed  from  the  sides  of  the  specimen,  and  the  moisture  disk  is 
cut  from  the  region  of  failure. 

Calculating  the  results:  The  formulae  used  in  calculating  the 
results  of  tests  on  endwise  compression  are  as  follows: 


P  P  D 

(2)   c  -  -ji  (4)   S  -  w 

C  =  crushing  strength,  pounds  per  square  inch. 

c  =  fibre  strength  at  elastic  limit,  pounds  per  square  inch. 
A  =  area  of  cross  section,  square  inches. 

I  =  distance  between  centres  of  collars,  inches. 
D  =  total  shortening  at  elastic  limit,  inches. 
V  =  volume  of  specimen,  cubic  inches. 
Remainder  of  legend  as  on  page  98. 

COMPRESSION    ACROSS    THE    GRAIN 

Apparatus:     An  ordinary  static  testing  machine,   a  bearing 
plate,  and  a  deflectometer  are  required.    (See  Fig.  35.) 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


FORM  512 
(Supersedes  Form  176) 

Project  No^_^__ 
•Working  Plan  No_>^-  — 


105 

Timber  Test  T> 


U.S.  DEPARTMENT  OF  AGRICULTURE 
FOREST  SERVICE 

Station    Madison  Date       3/fO/U 


Ship.  No_  2fJ Stick  No^/ 


t       Laborator    N 

Piece  J 

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Compression  in  inches 

FIG.  34. — Sample  log  sheet  of  an  endwise  compression  test  on  a  short  pine  column. 


106 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


Preparing  the  material:  Two  classes  of  specimens  are  used, 
namely,  (1)  sections  of  commercial  sizes  of  ties,  beams,  and  other 
timbers,  and  (2)  small,  clear  specimens  with  the  length  several 
times  the  width.  Sometimes  small  cubes  are  tested,  but  the  results 
are  hardly  applicable  to  conditions  in  practice.  In  (2)  the  sides 
are  surfaced  and  the  ends  squared.  The  specimens  are  then 
carefully  measured  and  weighed,  defects  noted,  rate  of  growth 


Photo  by  U.  S.  Forest  Service. 

FIG.   35. — Compression  across  the  grain.     Note  method  of  measuring  the 
deformation  by  means  of  a  deflectometer. 

and  proportion  of  late  wood  determined,  as  in  bending  tests. 
(See  page  95.)  After  the  test  a  moisture  section  is  cut  and 
weighed. 

Sketching :  Sketches  are  made  as  in  endwise  compression 
tests.  (See  page  102.) 

Adjusting  specimen  in  machine:  The  specimen  is  laid  horizon- 
tally upon  the  platform  of  the  machine  and  a  steel  bearing  plate 
placed  on  its  upper  surface  immediately  beneath  the  centre  of  the 
movable  head.  For  the  larger  specimens  this  plate  is  six  inches 
wide;  for  the  smaller  sizes,  two  inches  wide.  The  plate  in  all  cases 
projects  over  the  edges  of  the  test  piece,  and  in  no  case  should  the 
length  of  the  latter  be  less  than  four  times  the  width  of  the  plate. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


107 


Measuring  the  deformation:  The  compression  is  measured  by 
means  of  a  deflectometer  (see  Fig.  35),  which,  after  the  first  incre- 
ment of  load  is  applied,  is  adjusted  (by  means  of  a  small  set 
screw)  to  read  zero.  The  actual  downward  motion  of  the  movable 
head  (corresponding  to  the  compression  of  the  specimen)  is  mul- 
tiplied ten  times  on  the  scale  from  which  the  readings  are  made. 

Log  of  the  test:  The  load  is  applied  continuously  and  at  uni- 
form speed  (see  Speed  of  Testing  Machine,  p.  92),  until  well 
beyond  the  elastic  limit.  The  compression  readings  are  taken  at 
regular  load  increments  and  entered  on  the  cross-section  paper 
in  the  usual  way.  Usually  there  is  no  real 
maximum  load  in  this  case,  as  the  strength 
continually  increases  as  the  fibres  are  crushed 
more  compactly  together. 

Calculating  the  results:  Ordinarily  only 
the  fibre  stress  at  the  elastic  limit  (c)  is 
computed.  It  is  equal  to  the  load  at  elastic 
limit  (Pi)  divided  by  the  area  under  the 


plate  (B).  (c  =   -g- 


FIG.  36. — Vertical  sec- 
tion of  shearing  tool. 


SHEAR  ALONG  THE  GRAIN 

Apparatus:  An  ordinary  static  testing 
machine  and  a  special  tool  designed  for  pro- 
ducing single  shear  are  required.  (See  Figs. 
36  and  37.)  This  shearing  apparatus  con- 
sists of  a  solid  steel  frame  with  set  screws 
for  clamping  the  block  within  it  firmly  in 
a  vertical  position.  In  the  centre  of  the 
frame  is  a  vertical  slot  in  which  a  square- 
edged  steel  plate  slides  freely.  When  the  testing  block  is  in  posi- 
tion, this  plate  impinges  squarely  along  the  upper  surface  of  the 
tenon  or  lip,  which,  as  vertical  pressure  is  applied,  shears  off. 

Preparing  the  material:  The  specimens  are  usually  in  the  form 
of  small,  clear,  straight-grained  blocks  with  a  projecting  tenon  or 
lip  to  be  sheared  off.  Two  common  forms  and  sizes  are  shown 
in  Figure  38.  Part  of  the  blocks  are  cut  so  that  the  shearing 
surface  is  parallel  to  the  growth  rings,  or  tangential;  others  at 


Photo  by  U.  S.  Forest  Service. 

FIG.  37. — Front  view  of  shearing  tool  with  test  specimen  and  steel  plate  in 
position  for  testing. 


FIG.  38. — Two  forms  of  shear  test  specimens. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


109 


right  angles  to  the  growth  rings,  or  radial.  It  is  important  that 
the  upper  surface  of  the  tenon  or  lip  be  sawed  exactly  parallel 
to  the  base  of  the  block.  When  the  form  with  a  tenon  is  used 
the  under  cut  is  extended  a  short  distance  horizontally  into  the 
block  to  prevent  any  compression  from  below. 

In  designing  a  shearing  specimen  it  is  necessary  to  take  into 
consideration  the  proportions  of  the  area  of  shear,  since,  if  the 


Photo  by  U .  S.  Forest  Service. 

FIG.  39. — Making  a  shearing  test. 

length  of  the  portion  to  be  sheared  off  is  too  great  in  the  direction 
of  the  shearing  face,  failure  would  occur  by  compression  before 
the  piece  would  shear.  Inasmuch  as  the  endwise  compressive 
strength  is  sometimes  not  more  than  five  times  the  shearing 
strength,  the  shearing  surface  should  be  less  than  five  times  the 
surface  to  which  the  load  is  applied.  This  condition  is  fulfilled 
in  the  specimens  illustrated. 

Shearing  specimens  are  frequently  cut  from  beams  after  test- 
ing. In  this  case  the  specific  gravity  (dry),  proportion  of  late 
wood,  and  rate  of  growth  are  assumed  to  be  the  same  as  already 


110  THE    MECHANICAL   PROPERTIES    OF   WOOD 

recorded  for  the  beams.  In  specimens  not  so  taken,  these  quan- 
tities are  determined  in  the  usual  way.  The  sheared-off  portion 
is  used  for  a  moisture  section. 

Adjusting  specimen  in  machine:  The  test  specimen  is  placed  in 
the  shearing  apparatus  with  the  tenon  or  lip  under  the  sliding 
plate,  which  is  centred  under  the  movable  head  of  the  machine. 
(See  Fig.  39.)  In  order  to  reduce  to  a  minimum  the  friction  due 
to  the  lateral  pressure  of  the  plate  against  the  bearings  of  the 
slot,  the  apparatus  is  sometimes  placed  upon  several  parallel  steel 
rods  to  form  a  roller  base.  A  slight  initial  load  is  applied  to  take 
up  the  lost  motion  of  the  machinery,  and  the  beam  balanced. 

Log  of  the  test:  The  load  is  applied  continuously  and  at  a  uni- 
form rate  until  failure,  but  no  deformations  are  measured.  The 
points  noted  are  the  maximum  load  and  the  length  of  time  required 
to  reach  it.  Sketches  are  made  of  the  failure.  If  the  failure  is 
not  pure  shear  the  test  is  culled. 

The  shearing  strength  per  square  inch  is  found  by  dividing  the 

maximum  load  by  the  cross-sectional  area.     (Q  =  — p  j 

IMPACT   TEST 

Apparatus:  There  are  several  types  of  impact  testing  ma- 
chines.* One  of  the  simplest  and  most  efficient  for  use  with  wood 
is  illustrated  in  Figure  40.  The  base  of  the  machine  is  7  feet  long, 
2.5  feet  wide  at  the  centre,  and  weighs  3,500  pounds.  Two  up- 
right columns,  each  8  feet  long,  act  as  guides  for  the  striking 
head.  At  the  top  of  the  column  is  the  hoisting  mechanism  for 
raising  or  lowering  the  striking  weights.  The  power  for  operating 
the  machine  is  furnished  by  a  motor  set  on  the  top.  The  hoisting 
mechanism  is  all  controlled  by  a  single  operating  lever,  shown  on 
the  side  of  the  column,  whereby  the  striking  weight  may  be  raised, 
lowered,  or  stopped  at  the  will  of  the  operator.  There  is  an  auto- 
matic safety  device  for  stopping  the  machine  when  the  weight 
reaches  the  top. 

The  weight  is  lifted  by  a  chain,  one  end  of  which  passes  over  a 

*  For  description  of  U.  S.  Forest  Service  automatic  and  autographic  impact 
testing  machine,  see  Proc.  Am.  Soc.  for  Testing  Materials,  Vol.  VIII,  1908, 
pp.  538-540. 


FIG.  40. — Impact  testing  machine. 


112  THE   MECHANICAL   PROPERTIES   OF  WOOD 

sprocket  wheel  in  the  hoisting  mechanism.  On  the  lower  end  of 
the  chain  is  hung  an  electro-magnet  of  sufficient  magnetic  strength 
to  support  the  heaviest  striking  weights.  When  it  is  desired  to  drop 
the  striking  weight  the  electric  current  is  broken  and  reversed  by 
means  of  an  automatic  switch  and  current  breaker.  The  height 
of  drop  may  be  regulated  by  setting  at  the  desired  height  on  one 
of  the  columns  a  tripping  pin  which  throws  the  switch  on  the 
magnet  and  so  breaks  and  reverses  the  current. 

There  are  four  striking  weights,  weighing  respectively  50,  100, 
250,  and  500  pounds,  any  one  of  which  may  be  used,  depending 
upon  the  desired  energy  of  blow.  When  used  for  compression 
tests  a  flat  steel  head  six  inches  in  diameter  is  screwed  into  the 
lower  end  of  the  weight.  For  transverse  tests,  a  well-rounded 
knife  edge  is  screwed  into  the  weight  in  place  of  the  flat  head. 
Knife  edges  for  supporting  the  ends  of  the  specimen  to  be  tested, 
are  securely  bolted  to  the  base  of  the  machine. 

The  record  of  the  behavior  of  the  specimen  at  time  of  impact 
is  traced  upon  a  revolving  drum  by  a  pencil  fixed  in  the  striking 
head.  (See  Fig.  41.)  When  a  drop  is  made  the  pencil  comes  in 
contact  with  the  drum  and  is  held  in  place  by  a  spring.  The 
drum  is  revolved  very  slowly;  either  automatically  or  by  hand. 
The  speed  of  the  drum  can  be  recorded  by  a  pencil  in  the  end  of  a 
tuning  fork  which  gives  a  known  number  of  vibrations  per  second. 

One  size  of  this  machine  will  handle  specimens  for  transverse 
tests  9  inches  wide  and  6-foot  span;  the  other,  12  inches  wide  and 
8-foot  span.  For  compression  tests  a  free  fall  of  about  6.5  feet  may 
be  obtained.  For  transverse  tests  the  fall  is  a  little  less,  depending 
upon  the  size  of  the  specimen. 

The  machine  is  calibrated  by  dropping  the  hammer  upon  a 
copper  cylinder.  The  axial  compression  of  the  plug  is  noted. 
The  energy  used  in  static  tests  to  produce  this  axial  compression 
under  stress  in  a  like  piece  of  metal  is  determined.  The  external 
energy  of  the  blow  (i.e.,  the  weight  of  the  hammer  X  the  height 
of  drop)  is  compared  with  the  energy  used  in  static  tests  at  equal 
amounts  of  compression.  For  instance: 

Energy  delivered,  impact  test   .       .     35,000  inch-pounds 
Energy  computed  from  static  test  .     26,400 
Efficiency  of  blow  of  hammer  ,       .     75.3  per  cent. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


113 


Preparing  the  material:  The  material  used  in  making  impact 
tests  is  of  the  same  size  and  prepared  in  the  same  way  as  for  static 
bending  and  compression  tests.  Bending  in  impact  tests  is  more 
commonly  used  than  compression,  and  small  beams  with  28-inch 
span  are  usually  employed. 

Method:     In  making  an  impact  bending  test  the  hammer  is 


Complete  failure  by  tension  Wrinkling  on  top 

FIG.  41. — Drum  record  of  impact  bending  test. 

allowed  to  rest  upon  the  specimen  and  a  zero  or  datum  line  is 
drawn.  The  hammer  is  then  dropped  from  increasing  heights  and 
drum  records  taken  until  first  failure.  The  first  drop  is  one  inch 
and  the  increase  is  by  increments  of  one  inch  until  a  height  of 
ten  inches  is  reached,  after  which  increments  of  two  inches  are  used 
until  complete  failure  occurs  or  6-inch  deflection  is  secured. 


114  THE   MECHANICAL   PROPERTIES   OF  WOOD 

The  50-pound  hammer  is  used  when  with  drops  up  to  68  inches 
it  is  reasonably  certain  it  will  produce  complete  failure  or  6-inch 
deflection  in  the  case  of  all  specimens  of  a  species;  for  all  other 
species  a  100-pound  hammer  is  used. 

Results:  The  tracing  on  the  drum  (see  Fig.  41)  represents 
the  actual  deflection  of  the  stick  and  the  subsequent  rebounds  for 
each  drop.  The  distance  from  the  lowest  point  in  each  case  to  the 
datum  line  is  measured  and  its  square  in  tenths  of  a  square  inch 
entered  as  an  abscissa  on  cross-section  paper,  with  the  height  of 
drop  in  inches  as  the  ordinate.  The  elastic  limit  is  that  point  on 
the  diagram  where  the  square  of  the  deflection  begins  to  increase 
more  rapidly  than  the  height  of  drop.  The  difference  between  the 
datum  line  and  the  final  resting  point  after  each  drop  represents 
the  set  the  material  has  received. 

The  formulae  used  in  calculating  the  results  of  impact  tests  in 
bending  when  the  load  is  applied  at  the  centre  up  to  the  elastic 
limit  are  as  follows: 

3WHI  F_FSP  «WJ1 

Dbh2  (2)    E  ~  ~6£T  ~   Ib  h 

H  =  height  of  drop  of  hammer,  including  deflection, 

inches. 
S  =  modulus  of  elastic  resilience,  inch-pounds  per 

cubic  inch. 

W  =  weight  of  hammer,  pounds. 
Remainder  of  legend  as  on  page  98. 

HARDNESS   TEST!   ABRASION   AND    INDENTATION 

Abrasion:  The  machine  used  by  the  U.  S.  Forest  Service  is  a 
modified  form  of  the  Dorry  abrasion  machine.  (See  Fig.  42.) 
Upon  the  revolving  horizontal  disk  is  glued  a  commercial  sand- 
paper, known  as  garnet  paper,  which  is  commonly  employed  in 
factories  in  finishing  wood. 

A  small  block  of  the  wood  to  be  tested  is  fixed  in  one  clamp  and 
a  similar  block  of  some  wood  chosen  as  a  standard,  as  sugar  maple, 
at  10  per  cent  moisture,  in  the  opposite,  and  held  against  the 
same  zone  of  sandpaper  by  a  weight  of  26  pounds  each.  The 
size  of  the  section  under  abrasion  for  each  specimen  is  2"  X  2". 


Photo  by  U.  S.  Forest  Service. 

FIG.  42. — Abrasion  machine  for  testing  the  wearing  qualities  of  woods. 


116 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


The  conditions  for  wear  are  the  same  for  both  specimens.     The 
speed  of  rotation  is  68  revolutions  a  minute. 

The  test  is  continued  until  the  standard  specimen  is  worn  a 
specified  amount,  which  varies  with  the  kind  of  wood  under  test. 
A  comparison  of  the  wear  of  the  two  blocks  affords  a  fair  idea 
of  their  relative  resistance  to  abrasion. 

Another  method  makes  use  of  a  sand  blast  to  abrade  the 

woods  and  is  the  one  em- 
ployed in  New  South 
Wales.*  The  apparatus 
consists  essentially  of  a 
nozzle  through  which  sand 
can  be  propelled  at  a  high 
velocity  against  the  test 
specimen  by  means  of  a 
steam  jet. 

The  wood  to  be  tested 
is  cut  into  blocks  3"  X  3" 
XI',  and  these  are  weighed 
to  the  nearest  grain  just 
before  placing  in  the  ap- 
paratus. Steam  from  the 
boiler  at  a  pressure  of 
about  43  pounds  per  square 
inch  is  ejected  from  a  nozzle 
in  such  a  way  that  particles 
of  fine  quartz  sand  are 
caught  up  and  thrown 

SECTION  violently  against  the  block 

which  is  being  rotated. 
Only  superheated  steam 
strikes  the  block,  thus  leaving  the  wood  dry.  The  test  is  con- 
tinued for  two  minutes,  after  which  the  specimen  is  removed 
and  immediately  weighed. 

By     comparison     with    the   original    weight    the    loss    from 

*  See  Warren,  W.  H. :  The  strength,  elasticity,  and  other  properties  of 
New  South  Wales  hardwood  timbers.  Dept.  For.,  N.  S.  W.,  Sydney,  1911, 
pp.  88-95. 


FIG.  43. — Design  of  tool  for   testing   the 
hardness  of  woods  by  indentation. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


117 


abrasion  is  determined,  and  by  comparison  with  a  certain  wood 
chosen  as  a  standard,  a  coefficient  of  wear-resistance  can  be 
obtained.  The  amount  of  wear  will  vary  more  or  less  according 
to  the  surface  exposed,  and  in  these  tests  quarter-sawed  material 
was  used  with  the  edge  grain  to  the  blast. 

Indentation:  The  tool  used  for  this  test  consists  of  a  punch 
with  a  hemispherical  end  or  steel  ball  having  a  diameter  of  0.444 
inch,  giving  a  surface  area  of 
one-fourth  square  inch.  It  is 
fitted  with  a  guard  plate, 
which  works  loosely  until  the 
penetration  has  progressed 
to  a  depth  of  0.222  inch, 
whereupon  it  tightens.  (See 
Fig.  43.)  The  effect  is  that 
of  sinking  a  ball  half  its 
diameter  into  the  specimen. 
This  apparatus  is  fitted  into 
the  movable  head  of  the 
static  testing  machine. 

The  wood  to  be  tested  is 
cut  square  with  the  grain 
into  rectangular  blocks 
measuring  2"  X  2"  X  6".  A 
block  is  placed  on  the  plat- 
form and  the  end  of  the 
punch  forced  into  the  wood 
at  the  rate  of  0.25  inch  per 
minute.  The  operator  keeps  FlG  44._Design  of  tool  for  cleavage  test. 
moving  the  small  handle  of 

the  guard  plate  back  and  forth  until  it  tightens.  At  this  instant 
the  load  is  read  and  recorded. 

Two  penetrations  each  are  made  on  the  tangential  and  radial 
surfaces,  and  one  on  each  end  of  every  specimen  tested. 

In  choosing  the  places  on  the  block  for  the  indentations,  effort 
should  be  made  to  get  a  fair  average  of  heartwood  and  sapwood, 
fine  and  coarse  grain,  early  and  late  wood. 

Another  method  of  testing  by  indentation  involves  the  use 


118 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


of  a  right-angled  cone  instead  of  a  ball.    For  details  of  this  test  as 
used  in  New  South  Wales  see  loc.  cit.,  pp.  86-87. 

CLEAVAGE    TEST 

A  static  testing  machine  and  a  special  cleavage  testing  device 
are  required.  (See  Fig.  44.)  The  latter  consists  essentially  of  two 

hooks,  one  of  which  is  suspended  from 
the  centre  of  the  top  of  the  cage, 
the  other  extended  above  the  movable 
head. 

The  specimens  are  2"  X  2"  X  3.75". 
At  one  end  a  one-inch  hole  is  bored, 
with  its  centre  equidistant  from  the  two 
sides  and  0.25  inch  from  the  end.  (See 
Fig.  45.)  This  makes  the  cross  section 
to  be  tested  2"  X  3".  Some  of  the 
blocks  are  cut  radially  and  some  tan- 
gentially,  as  indicated  in  the  figure. 

The  free  ends  of  the  hooks  are  fitted 
into  the  notch  in  the  end  of  the  specimen. 
The  movable  head  of  the  machine  is  then 
made  to  descend  at  the  rate  of  0.25  inch 
per  minute,  pulling  apart  the  hooks  and 
splitting  the  block.  The  maximum  load 
only  is  taken  and  the  result  expressed  in 
pounds  per  square  inch  of  width.  A 
piece  one-half  inch  thick  is  split  off 
parallel  to  the  failure  and  used  for 
moisture  determination. 


Tangential 


Radial 

FIG.  45. — Design  of  cleav- 
age test  specimen. 


TENSION  TEST  PARALLEL  TO  THE  GRAIN 

Since  the  tensile  strength  of  wood 
parallel  to  the  grain  is  greater  than 
the  compressive  strength,  and  exceed- 
ingly greater  than  the  shearing  strength, 
it  is  very  difficult  to  make  satisfactory 
tension  tests,  as  the  head  and  shoulders 
of  the  test  specimen  (which  is  subjected 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


119 


to  both  compression  and  shear)  must  be  stronger  than  the  por- 
tion subjected  to  a  pure  tensile  stress. 

Various  designs  of  test  specimens  have  been  made.  The  one 
first  employed  by  the  Division  of  Forestry  *  was  prepared  as 
follows:  Sticks  were  cut  measuring  1.5"  X  2.5"  X  16".  The 
thickness  at  the  centre  was  then  reduced  to  three-eighths  of  an 


No.2 


FIG.  46. — Designs  of  tension  test  specimens  used  in  United  States. 

inch  by  cutting  out  circular  segments  with  a  band  saw.  This  left 
a  breaking  section  of  2.5"  X  0.375".  Care  was  taken  to  cut  the 
specimen  as  nearly  parallel  to  the  grain  as  possible,  so  that  its 
failure  would  occur  in  a  condition  of  pure  tension.  The  specimen 
was  then  placed  between  the  plane  wedge-shaped  steel  grips  of 
the  cage  and  the  movable  head  of  the  static  machine  and  pulled  in 
two.  Only  the  maximum  load  was  recorded.  (See  Fig.  46,  No.  1.) 

*Bul.  No.  8:     Timber  physic?.     Part  II.,  1893,  p.  7. 


120 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


The  difficulty  of  making  such  tests  compared  with  the  minor 
importance  of  the  results  is  so  great  that  they  are  at  present 
omitted  by  the  U.  S.  Forest  Service.  A  form  of  specimen  is  sug- 
gested, however,  and  is  as  follows:  "  A  rod  of  wood  about  one  inch 
in  diameter  is  bored  by  a  hollow  drill  from  the  stick  to  be  tested. 
The  ends  of  this  rod  are  inserted  and  glued  in  corresponding  holes 
in  permanent  hardwood  wedges.  The  specimen  is  then  submitted 
to  the  ordinary  tension  test.  The  broken  ends  are  punched  from 
the  wedges."  *  (See  Fig.  46,  No.  2.) 

The  form  used  by  the  Department  of  Forestry  of  New  South 
Wales  t  is  as  shown  in  Fig.  47.  The  specimen  has  a  total  length 
of  41  inches  and  is  circular  in  cross  section.  On  each  end  is  a  head 
4  inches  in  diameter  and  7  inches  long.  Below  each  head  is  a 


FIG.  47. — Design  of  tension  test  specimen  used  in  New  South  Wales. 

shoulder  8.5  inches  long,  which  tapers  from  a  diameter  of  2.75 
inches  to  1.25  inches.  In  the  middle  is  a  cylindrical  portion 
1.25  inches  in  diameter  and  10  inches  long. 

In  making  the  test  the  specimen  is  fitted  in  the  machine,  and 
an  extensometer  attached  to  the  middle  portion  and  arranged 
to  record  the  extension  between  the  gauge  points  8  inches  apart. 
The  area  of  the  cross  section  then  is  1.226  square  inches,  and  the 
tensile  strength  is  equal  to  the  total  breaking  load  applied  divided 
by  this  area. 


TENSION   TEST   AT    RIGHT   ANGLES    TO    THE    GRAIN 

A  static  testing  machine  and  a  special  testing  device  (see  Fig. 
48)   are  required.     The  latter  consists  essentially  of  two  double 

*  Cir.  38:     Instructions  to  engineers  of  timber  tests,  1906,  p.  24. 
t  Warren,  W.  H.:   The  strength,  elasticity,  and  other  properties  of  New- 
South   Wales   hardwood  timbers,  1911,  pp.  .58-62. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


121 


hooks  or  clamps,  one  of  which  is  suspended  from  the  centre  of  the 
top  of  the  cage,  the  other  extended  above  the  movable  head. 
The  specimens  are  2"  X  2"  X  2.5".     At  each  end  a  one-inch 


Tester 


FIG.  48. — Design  of  tool  and  specimen  for  testing  tension  at  right  angles 
to  the  grain. 

hole  is  bored  with  its  centre  equidistant  from  the  two  sides  and 
0.25  inch  from  the  ends.  This  makes  the  cross  section  to  be  tested 
I"  X  2". 

The  free  ends  of  the  clamps  are  fitted  into  the  notches  in  the 


122 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


ends  of  the  specimen.  The  movable  head  of  the  machine  is  then 
made  to  descend  at  the  rate  of  0.25  inch  per  minute,  pulling 
the  specimen  in  two  at  right  angles  to  the  grain.  The  maxi- 
mum load  only  is  taken  and  the  result  expressed  in  pounds  per 


Photo  by  U.  S.  Forest  Service. 

FIG.  49. — Making  a  torsion  test  on  hickory. 

inch  of  width.     A  piece  one-half  inch  thick  is  split  off  parallel  to 
the  failure  and  used  for  moisture  determination. 


TORSION    TEST* 

Apparatus:  The  torsion  test  is  made  in  a  Riehle-Miller  tor- 
sional  testing  machine  or  its  equivalent.  (See  Fig.  49.) 

Preparation  of  material:  The  test  pieces  are  cylindrical,  1.5 
inches  in  diameter  and  18  inches  gauge  length,  with  squared  ends 
4  inches  long  joined  to  the  cylindrical  portion  with  a  fillet.  The 
dimensions  are  carefully  measured,  and  the  usual  data  obtained 
in  regard  to  the  rate  of  growth,  proportion  of  late  wood,  location 

Wood  is  so  seldom  subjected  to  a  pure  stress  of  this  kind  that  the  torsion 
test  is  usually  omitted. 


THE  MECHANICAL  PROPERTIES  OF  WOOD       123 

and  kind  of  defects.  The  weight  of  the  cylindrical  portion  of  the 
specimen  is  obtained  after  the  test. 

Making  the  test:  After  the  specimen  is  fitted  in  the  machine  the 
load  is  applied  continuously  at  the  rate  of  22°  per  minute.  A 
troptometer  is  used  in  measuring  the  deformation.  Readings  are 
made  until  failure  occurs,  the  points  being  entered  on  the  cross- 
section  paper.  The  character  of  the  failure  is  described.  Moisture 
determinations  are  made  by  the  disk  method. 

Results:  The  conditions  of  ultimate  rupture  due  to  torsion 
appear  not  to  be  governed  by  definite  mathematical  laws;  but 
where  the  material  is  not  overstrained,  laws  may  be  assumed 
which  are  sufficiently  exact  for  practical  cases.  The  formulae 
commonly  used  for  computations  are  as  follows: 


a  =  angle  measured  by  troptometer  at  elastic  limit,  in 

degrees. 

c  =  diameter  of  specimen,  inches. 
/  =  gauge  length  of  specimen,  inches. 
G  =  modulus  of  elasticity  in  shear  across  the   grain, 

pounds  per  square  inch. 

M  =  moment  of  torsion  at  elastic  limit,  inch-pounds. 
T  =  outer  fibre  torsional  stress  at  elastic  limit,  pounds 
per  square  inch. 

SPECIAL   TESTS 

Spike-pulling  Test 

Spike-pulling  tests  apply  to  problems  of  railroad  maintenance, 
and  the  results  are  used  to  compare  the  spike-holding  powers  of 
various  woods,  both  untreated  and  treated  with  different  preserv- 
atives, and  the  efficiency  of  various  forms  of  spikes.  Special  tests 
are  also  made  in  which  the  spike  is  subjected  to  a  transverse  load 
applied  repetitively  by  a  blow. 

For  details  of  tests  and  results  see: 

Cir.  38,  U.S.F.S.  :  Instructions  to  engineers  of  timber  tests,  p.  26. 
Cir.  46,  U.S.F.S.  :  Holding  force  of  railroad  spikes  in  wooden  ties. 
Bui.  118,  U.S.F.S.:  Prolonging  the  life  of  cross-ties,  pp,  37-40. 


124  THE    MECHANICAL   PROPERTIES   OF   WOOD 

Packing  Boxes 

Special  tests  on  the  strength  of  packing  boxes  of  various 
woods  have  been  made  by  the  U.  S.  Forest  Service  to  determine 
the  merits  of  different  kinds  of  woods  as  box  material  with  the 
view  of  substituting  new  kinds  for  the  more  expensive  ones  now 
in  use.  The  methods  of  tests  consisted  in  applying  a  load  along 
the  diagonal  of  a  box,  an  action  similar  to  that  which  occurs  when 
a  box  is  dropped  on  one  of  its  corners.  The  load  was  measured 
at  each  one-fourth  inch  in  deflection,  and  notes  were  made  of  the 
primary  and  subsequent  failures. 

For  details  of  tests  and  results,  see : 

Cir.  47,     U.S.F.S.:   Strength  of  packing  boxes  of  various  woods. 
Cir.  214,  U.S.F.S.:   Tests  of  packing  boxes  of  various  forms. 

Vehicle  and  Implement  Woods 

Tests  were  made  by  the  U.  S.  Forest  Service  to  obtain  a  better 
knowledge  of  the  mechanical  properties  of  the  woods  at  present 
used  in  the  manufacture  of  vehicles  and  implements  and  of  those 
which  might  be  substituted  for  them.  Tests  were  made  upon  the 
following  materials:  hickory  buggy  spokes  (see  Fig.  o,  page  11); 
hickory  and  red  oak  buggy  shafts;  wagon  tongues;  Douglas  fir 
and  southern  pine  cultivator  poles. 

Details  of  the  tests  and  results  may  be  found  in : 
Cir.  142,  U.S.F.S.:    Tests  on  vehicle  and  implement  woods. 

Cross-arms 

In  tests  by  the  U.  S.  Forest  Service  on  cross-arms  a  special 
apparatus  was  devised  in  which  the  load  was  distributed  along  the 
arm  as  in  actual  practice.  The  load  was  applied  by  rods  passing 
through  the  pinholes  in  the  arms.  Nuts  on  these  rods  pulled  down 
on  the  wooden  bearing-blocks  shaped  to  fit  the  upper  side  of  the 
arm.  The  lower  ends  of  these  rods  were  attached  to  a  system  of 
equalizing  levers,  so  arranged  that  the  load  at  each  pinhole  would 
be  the  same.  In  all  the  tests  the  load  was  applied  vertically  by 
means  of  the  static  machine. 
See  Cir.  204,  U.S.F.S.:  Strength  tests  of  cross-arms. 


THE  MECHANICAL  PROPERTIES  OF  WOOD       125 

Other  Tests 

Many  other  kinds  of  tests  are  made  as  occasion  demands. 
One  kind  consists  of  barrels  and  liquid  containers,  match-boxes, 
and  explosive  containers.  These  articles  are  subjected  to  shocks 
such  as  they  would  receive  in  transit  and  in  handling,  and  also  to 
hydraulic  pressure. 

One  of  the  most  important  tests  from  a  practical  standpoint  is 
that  of  built-up  structures  such  as  compounded  beams  composed 
of  small  pieces  bolted  together,  mortised  joints,  wooden  trusses, 
etc.  Tests  of  this  kind  can  best  be  worked  out  according  to  the 
specific  requirements  in  each  case. 


APPENDIX 

SAMPLE    WORKING    PLAN    OF    THE 
U.    S.    FOREST    SERVICE 


MECHANICAL  PROPERTIES  OF  WOODS  GROWN  IN  THE 
UNITED   STATES 

Working  Plan  No.  124 


PURPOSE    OF    WORK 

IT  is  the  general  purpose  of  the  work  here  outlined  to  provide : 
(a)    Reliable  data  for  comparing  the  mechanical  properties  of 
various  species; 

(6)    Data  for  the  establishment  of  correct  strength  functions  or 
working  stresses; 

(c)    Data  upon  which  may  be  based  analyses  of  the  influence 
on  the  mechanical  properties  of  such  factors  as : 
Locality ; 

Distance  of  timber  from  the  pith  of  the  tree; 
Height  of  timber  in  the  tree; 

Change  from  the  green  to  the  air-dried  condition,  etc. 
The  mechanical  properties  which  will  be  considered  and  the 
principal  tests  used  to  determine  them  are  as  follows: 
Strength  and  stiffness — 
Static  bending; 
Compression  parallel  to  grain; 
Compression  perpendicular  to  grain; 
Shear. 
Toughness — 

Impact  bending; 
Static  bending; 

Work  to  maximum  load  and  total  work. 
127 


128  THE    MECHANICAL    PROPERTIES    OF    WOOD 

Cleavability— 

Cleavage  test. 
Hardness — 

Modification  of  Janka  ball  test  for  surface  hardness. 

MATERIAL 

Selection  and  Number  of  Trees 

The  material  will  be  from  trees  selected  in  the  forest  by  one 
qualified  to  determine  the  species.  From  each  locality,  three  to 
five  dominant  trees  of  merchantable  size  and  approximately 
average  age  will  be  so  chosen  as  to  be  representative  of  the  domi- 
nant trees  of  the  species.  Each  species  will  eventually  be  repre- 
sented by  trees  from  five  to  ten  localities.  These  localities  will  be 
so  chosen  as  to  be  representative  of  the  commercial  range  of  the 
species.  Trees  from  one  to  three  localities  will  be  used  to  repre- 
sent each  species  until  most  of  the  important  species  have  been 
tested. 

The  16-foot  butt  log  will  be  taken  from  each  tree  selected  and 
the  entire  merchantable  bole  of  one  average  tree  for  each  species. 

Field  Notes  and  Shipping  Instructions 

Field  notes  as  outlined  in  Form  — a  Shipment  Description, 
Manual  of  the  Branch  of  Products,  will  be  fully  and  carefully 
made  by  the  collector.  The  age  of  each  tree  selected  will  be 
recorded  and  any  other  information  likely  to  be  of  interest  or  im- 
portance will  also  be  made  a  part  of  these  field  notes.  Each  log 
will  have  the  bark  left  on.  It  will  be  plainly  marked  in  accordance 
with  directions  given  under  Detailed  Instructions.  All  material 
will  be  shipped  to  the  laboratory  immediately  after  being  cut.  No 
trees  will  be  cut  until  the  collector  is  notified  that  the  laboratory 
is  ready  to  receive  the  material. 

DETAILED    INSTRUCTIONS 

Part  of  Tree  to  be  Tested 

(a)  For  determining  the  value  of  tree  and  locality  and  the 
influence  on  the  mechanical  properties  of  distance  from  the  pith, 
a  4-foot  bolt  will  be  cut  from  the  top  end  of  each  16-foot  butt  log. 


THE    MECHANICAL    PROPERTIES    OF    WOOD 


129 


North 


(6)  For  investigating  the  variation  of  properties  with  the 
height  of  timber  in  the  tree,  all  the  logs  from  one  average  tree  will 
be  used. 

(c)  For  investigating  the  effect  of  drying  the  wood,  the  bolt 
next  below  that  provided  for  in  (a)  will  be  used  in  the  case  of  one 
tree  from  each  locality. 

Marking  and  Grouping  of  Material 

The  marking  will  be  standard  except  as  noted.  Each  log 
will  be  considered  a  "  piece."  The  piece  numbers  will  be  plainly 
marked  upon  the  butt  end  of  each  log  by  the  collector.  The  north 
side  of  each  log  will  also  be  marked. 

When  only  one  bolt  from  a  tree  is  used  it  will  be  designated 
by  the  number  of  the  log  from  which  it  is  cut.  Whenever  more 
than  one  bolt  is  taken  from 
a  tree,  each  4-foot  bolt  or 
length  of  trunk  will  be  given 
a  letter  (mark),  a,  b,  c,  etc., 
beginning  at  the  stump. 

All  bolts  will  be  sawed  in- 
to 21A"  X  2l/2"  sticks  and 
the  sticks  marked  according 
to  the  sketch,  Fig.  50.  The 
letters  N,  E,  S,  and  W  indi- 
cate the  cardinal  points 
when  known;  when  these 
are  unknown,  H,  K,  L,  and 
M  will  be  used.  Thus,  No, 
J\  8,  S7,  M4  are  stick  num- 
bers, the  letter  being  a  part 
of  the  stick  number. 

Only  straight-grained  specimens,  free  from  defects  which  will 
affect  their  strength,  will  be  tested. 

Care  of  Material 

Xo  material  will  be  kept  in  the  bolt  or  log  long  enough  to  be 
damaged  or  disfigured  by  checks,  rot,  or  stains. 


FIG.  50. — Method  of  cutting  and  marking 
test  specimens. 


130  THE   MECHANICAL    PROPERTIES   OF   WOOD 

Green  material:  The  material  to  be  tested  green  will  be  kept 
in  a  green  state  by  being  submerged  in  water  until  near  the  time 
of  test.  It  will  then  be  surfaced,  sawed  to  length,  and  stored  in 
damp  sawdust  at  a  temperature  of  70°  F.  (as  nearly  as  practicable) 
until  time  of  test.  Care  should  be  taken  to  avoid  as  much  as  pos- 
sible the  storage  of  green  material  in  any  form. 

Air-dry  material:  The  material  to  be  air-dried  will  be  cut 
into  sticks  2j^"  X  21A"  X  4'.  The  ends  of  these  sticks  will  be 
paraffined  to  prevent  checking.  This  material  will  be  so  piled  as 
to  leave  an  air  space  of  at  least  one-half  inch  on  each  side  of  each 
stick,  and  in  such  a  place  that  it  will  be  protected  from  sunshine, 
rain,  snow,  and  moisture  from  the  ground.  The  sticks  will  be 
surfaced  and  cut  to  length  just  previous  to  test. 

Order  of  Tests 

The  order  of  tests  in  all  cases  will  be  such  as  to  eliminate  so 
far  as  possible  from  the  comparisons  the  effect  of  changes  of  con- 
dition of  the  specimens  due  to  such  factors  as  storage  and  weather 
conditions. 

The  material  used  for  determining  the  effect  of  height  in  tree 
will  be  tested  in  such  order  that  the  average  time  elapsing  from 
time  of  cutting  to  time  of  test  will  be  approximately  the  same  for 
all  bolts  from  any  one  tree. 

Tests  on  Green  Material 

The  tests  on  all  bolts,  except  those  from  which  a  comparison 
of  green  and  dry  timber  is  to  be  gotten,  will  be  as  follows: 

Static  bending :  One  stick  from  each  pair.  A  pair  consists  of 
two  adjacent  sticks  equidistant  from  the  pith,  as  N7  and  JV8,  or 
#5  and  #6. 

Impact  bending:  Four  sticks;  one  to  be  taken  from  near  the 
pith;  one  from  near  the  periphery;  and  two  representative  of  the 
cross  section. 

Compression  parallel  to  grain:  One  specimen  from  each  stick. 
These  will  be  marked  "  1  "  in  addition  to  the  number  of  the  stick 
from  which  they  are  taken. 

Compression  perpendicular  to  grain:  One  specimen  from  each 
of  50  per  cent  of  the  static  bending  sticks.  These  will  be  marked 


THE  MECHANICAL  PROPERTIES  OF  WOOD       131 

"  2  "  in  addition  to  the  number  of  the  stick  from  which  they 
are  cut. 

Hardness:  One  specimen  from  each  of  the  other  50  per  cent 
of  the  static  bending  sticks.  These  specimens  will  be  marked  "4." 

Shear:  Six  specimens  from  sticks  not  tested  in  bending  or 
from  the  ends  cut  off  in  preparing  the  bending  specimens.  Two 
specimens  will  be  taken  from  near  the  pith;  two  from  near  the 
periphery;  and  two  that  are  representative  of  the  average  growth. 
One  of  each  two  will  be  tested  in  radial  shear  and  the  other  in 
tangential  shear.  These  specimens  will  have  the  mark  "  3." 

Cleavage:  Six  specimens  chosen  and  divided  just  as  those  for 
shearing.  These  specimens  will  have  the  mark  "  5."  (For  sketches 
showing  radial  and  tangential  cleavage,  see  Fig.  45,  page  118.) 

When  it  is  impossible  to  secure  clear  specimens  for  all  of  the 
above  tests,  tests  will  have  precedence  in  the  order  in  which  they 
are  named. 

Tests  to  Determine  the  Effect  of  Air-drying 

These  tests  will  be  made  on  material  from  the  adjacent  bolts 
mentioned  in  "  c  "  under  Part  of  Tree  to  be  Tested.  Both 
bolts  will  be  cut  as  outlined  above.  One-half  the  sticks  from 
each  bolt  will  be  tested  green,  the  other  half  will  be  air-dried  and 
tested.  The  division  of  green  and  air-dry  will  be  according  to  the 
following  scheme  : 

STICK   NUMBERS 

Lower  bolt,     1,  4,  5,  8,  9,  ] 

lested 
etc.      > 

Upper  bolt,  2,  3,  6,  7,  10, 

Lower  bolt,  2,  3,  6,  7,  10,  1  Air-dried 

etc.       >      and 
Upper  bolt,     1,  4,  5,  8,  9,  j    tested 

All  green  sticks  from  these  two  bolts  will  be  tested  as  if  they 
were  from  the  same  bolt  and  according  to  the  plan  previously  out- 
lined for  green  material  from  single  bolts.  The  tests  on  the  air- 
dried  material  will  be  the  same  as  on  the  green  except  for  the 
difference  of  seasoning. 


132  THE    MECHANICAL    PROPERTIES    OF   WOOD 

The  material  will  be  tested  at  as  near  12  per  cent  moisture  as 
is  practicable.  The  approximate  weight  of  the  air-dried  speci- 
mens at  12  per  cent  moisture  will  be  determined  by  measuring 
while  green  20  per  cent  of  the  sticks  to  be  air-dried  and  assuming 
their  dry  gravity  to  be  the  same  as  that  of  the  specimens  tested 
green.  This  20  per  cent  will  be  weighed  as  often  as  is  necessary 
to  determine  the  proper  time  of  test. 

Methods  of  Test 

All  tests  will  be  made  according  to  Circular  38  except  in  case 
of  conflict  with  the  instructions  given  below: 

Static  bending:  The  tests  will  be  on  specimens  1"  X  2"  X  30" 
on  28-inch  span.  Load  will  be  applied  at  the  centre. 

In  all  tests  the  load-deflection  curve  will  be  carried  to  or  beyond 
the  maximum  load.  In  one-third  of  the  tests  the  load-deflection 
curve  will  be  continued  to  6-inch  deflection,  or  till  the  specimen 
fails  to  support  a  200-pound  load.  Deflection  readings  for  equal 
increments  of  load  will  be  taken  until  well  past  the  elastic  limit, 
after  which  the  scale  beam  will  be  kept  balanced  and  the  load  read 
for  each  0.1-inch  deflection.  The  load  and  deflection  at  first 
failure,  maximum  load  and  points  of  sudden  change,  will  be 
shown  on  the  curve  sheet  even  if  they  do  not  occur  at  one  of  the 
regular  load  or  deflection  increments. 

Impact  bending:  The  impact  bending  tests  will  be  on  speci- 
mens of  the  same  size  as  those  used  in  static  bending.  The  span 
will  be  28  inches. 

The  tests  will  be  by  increment  drop.  The  first  drop  will  be 
1  inch  and  the  increase  will  be  by  increments  of  1  inch  till  a  height 
of  10  inches  is  reached,  after  which  increments  of  2  inches  will  be 
used  until  complete  failure  occurs  or  6-inch  deflection  is  secured. 

A  50-pound  hammer  will  be  used  when  with  drops  up  to  68 
inches  it  is  practically  certain  that  it  will  produce  complete  failure 
or  6-inch  deflection  in  the  case  of  all  specimens  of  a  species.  For 
all  other  species,  a  100-pound  hammer  will  be  used. 

In  all  cases  drum  records  will  be  made  until  first  failure.  Also 
the  height  of  drop  causing  complete  failure  or  6-inch  deflection 
will  be  noted. 

Compression  parallel  to  grain:    This  test  will  be  on  specimens 


THE  MECHANICAL  PROPERTIES  OF  WOOD       133 

2"  X  2"  X  8"  in  size.  On  20  per  cent  of  these  tests  load-compression 
curves  for  a  6-inch  centrally  located  gauge  length  will  be  taken. 
Readings  will  be  continued  until  the  elastic  limit  is  well  passed. 
The  other  80  per  cent  of  the  tests  will  be  made  for  the  purpose  of 
obtaining  the  maximum  load  only. 

Compression  perpendicular  to  grain:  This  test  will  be  on 
specimens  2"  X2"  X6"  in  size.  The  bearing  plates  will  be  2  inches 
wide.  The  rate  of  descent  of  the  moving  head  will  be  0.024  inch 
per  minute.  The  load-compression  curve  will  be  plotted  to  0.1 
inch  compression  and  the  test  will  then  be  discontinued. 

Hardness:  The  tool  shown  in  Fig.  43,  page  116  (an  adapta- 
tion of  the  apparatus  used  by  the  German  investigator,  Janka) 
will  be  used.  The  rate  of  descent  of  the  moving  head  will  be  0.25 
inch  per  minute.  When  the  penetration  has  progressed  to  the 
point  at  which  the  plate  "  a  "  becomes  tight,  due  to  being  pressed 
against  the  wood,  the  load  will  be  read  and  recorded. 

Two  penetrations  will  be  made  on  a  tangential  surface,  two  on  a 
radial,  and  one  on  each  end  of  each  specimen  tested.  The  choice 
between  the  two  radial  and  between  the  two  tangential  surfaces 
and  the  distribution  of  the  penetrations  over  the  surfaces  will  be 
so  made  as  to  get  a  fair  average  of  heart  and  sap,  slow  and  fast 
growth,  and  spring  and  summer  wood.  Specimens  will  be  2"  X 
2"X6". 

Shear:  The  tests  will  be  made  with  a  tool  slight ly  modified 
from  that  shown  in  Circular  38.  The  speed  of  descent  of  head 
will  be  0.015  inch  per  minute.  The  only  measurements  to  be  made 
are  those  of  the  shearing  area.  The  offset  will  be  y$  inch.  Speci- 
mens will  be  2"X2"X2>^"  in  size.  (For  definition  of  offset  and 
form  of  test  specimen,  see  Fig.  38,  page  108.) 

Cleavage:  The  cleavage  tests  will  be  made  011  specimens  of 
the  form  and  size  shown  in  Fig.  45,  page  118.  The  apparatus  will 
be  as  shown  in  Fig.  44.  The  maximum  load  only  will  be  taken 
and  the  result  expressed  in  pounds  per  inch  of  width.  The  speed 
of  the  moving  head  will  be  0.25  inch  per  minute. 

Moisture  Determinations 

Moisture  determinations  will  be  made  on  all  specimens  tested 
except  those  to  be  photographed  or  kept  for  exhibit.  A  1-inch 


134  THE    MECHANICAL   PROPERTIES    OF   WOOD 

disk  will  be  cut  from  near  the  point  of  failure  of  bending  and  com- 
pression parallel  specimens,  from  the  portion  under  the  plate  in  the 
case  of  the  compression  perpendicular  specimens,  and  from  the 
centre  of  the  hardness  test  specimens.  The  beads  from  the  shear 
specimens  will  be  used  as  moisture  disks.  In  the  case  of  the 
cleavage  specimens  a  piece  ^2  inch  thick  will  be  split  off  parallel 
to  the  failure  and  used  as  a  moisture  disk. 

EECORDS 
All  records  will  be  standard. 

PHOTOGRAPHS 

Cross  Sections 

Just  before  cutting  into  sticks,  the  freshly  cut  end  of  at  least 
one  bolt  from  each  tree  will  be  photographed.  A  scale  of  inches 
will  be  shown  in  this  photograph. 

Specimens 

Three  photographs  will  be  made  of  a  group  consisting  of  four 
2"X2"X30"  specimens  chosen  from  the  material  from  each  local- 
ity. Two  of  these  specimens  will  be  representative  of  average 
growrth,  one  of  fast  and  one  of  slow  growth.  These  photographs 
will  show  radial,  tangential,  and  end  surfaces  for  each  specimen. 

Failures 

Typical  and  abnormal  failures  of  material  from  each  site  will 
be  photographed. 

Disposition  of  Material 

The  specimens  photographed  to  show  typical  and  abnormal 
failures  will  be  saved  for  purposes  of  exhibit  until  deemed  by  the 
person  in  charge  of  the  laboratory  to  be  of  no  further  value. 


SHRINKAGE  AND   SPECIFIC   GRAVITY 
Appendix  to  Working  Plan  124 


PURPOSE    OF   WORK 

It  is  the  purpose  of  this  work  to  secure  data  on  the  shrinkage 
and  specific  gravity  of  woods  tested  under  Project  124.  The  fig- 
ures to  be  obtained  are  for  use  as  average  working  values  rather 
than  as  the  basis  for  a  detailed  study  of  the  principles  involved. 

MATERIAL 

The  material  will  be  taken  from  that  provided  for  mechanical 
tests. 

RADIAL    AND    TANGENTIAL    SHRINKAGE 

Specimens 

Preparation:  Two  specimens  1  inch  thick,  4  inches  wide,  and 
1  inch  long  will  be  obtained  from  near  the  periphery  of  each  "  d  " 
bolt.  These  will  be  cut  from  the  sector-shaped  sections  left  after 
securing  the  material  for  the  mechanical  tests  or  from  disks  cut 
from  near  the  end  of  the  bolt.  They  will  be  taken  from  adjoining 
pieces  chosen  so  that  the  results  will  be  comparable  for  use  in 
determining  radial  and  tangential  shrinkage.  (When  a  disk  is 
used,  care  must  be  taken  that  it  is  green  and  has  not  been  affected 
by  the  shrinkage  and  checking  near  the  end  of  the  bolt.) 

One  of  these  specimens  will  be  cut  with  its  width  in  the  radial 
direction  and  will  be  used  for  the  determination  of  radial  shrinkage. 
The  other  will  have  its  width  in  the  tangential  direction  and  will 
be  used  for  tangential  shrinkage.  These  specimens  will  not  be 
surfaced. 

Marking:  The  shrinkage  specimens  will  retain  the  shipment 
and  piece  numbers  and  marks  of  the  bolts  from  which  they  are 
taken,  and  will  have  the  additional  mark  7R  or  IT  according  as 
their  widths  are  in  the  radial  or  tangential  direction. 

135 


136  THE    MECHANICAL    PROPERTIES    OF   WOOD 

Shrinkage  measurements:  The  shrinkage  specimens  will  be 
carefully  weighed  and  measured  soon  after  cutting.  Rings  per  inch, 
per  cent  sap,  and  per  cent  summer  wood  will  be  measured.  They 
will  then  be  air-dried  in  the  laboratory  to  constant  weight,  and 
afterward  oven-dried  at  100°  C.  (212°  F.),  when  they  will  again 
be  weighed  and  measured. 

VOLUMETRIC    SHRINKAGE    AND    SPECIFIC    GRAVITY 

Specimens 

Selection  and  preparation:  Four  2"  X  2"  X  6"  specimens  will 
be  cut  from  the  mechanical  test  sticks  of  each  "  d"  bolt;  also  from 
each  of  the  composite  bolts  used  in  getting  a  comparison  of  green 
and  air-dry.  One  of  these  specimens  will  be  taken  from  near  the 
pith  and  one  from  near  the  periphery;  the  other  two  will  be  repre- 
sentative of  the  average  growth  of  the  bolt.  The  sides  of  these 
specimens  will  be  surfaced  and  the  ends  smooth  sawn. 

Marking:  Each  specimen  will  retain  the  shipment,  piece,  and 
stick  numbers  and  mark  of  the  stick  from  which  it  is  cut,  and  will 
have  the  additional  mark  "  S." 

Manipulation:  Soon  after  cutting,  each  specimen  \vill  be 
weighed  and  its  volume  will  be  determined  by  the  method  described 
below.  The  rings  per  inch  and  per  cent  summer  wood,  where 
possible,  will  be  determined,  and  a  carbon  impression  of  the  end 
of  the  specimen  made.  It  will  then  be  air-dried  in  the  laboratory 
to  a  constant  weight  and  afterward  oven-dried  at  100°  C.  When 
dry,  the  specimen  will  be  taken  from  the  oven,  weighed,  and  a 
carbon  impression  of  its  end  made.  While  still  warm  the  speci- 
men will  be  dipped  in  hot  paramne.  The  volume  will  then  be 
determined  by  the  following  method: 

On  one  pan  of  a  pair  of  balances  is  placed  a  container  having 
in  it  water  enough  for  the  complete  submersion  of  the  test  speci- 
men. This  container  and  water  is  balanced  by  weights  placed 
on  the  other  scale  pan.  The  specimen  is  then  held  completely 
submerged  and  not  touching  the  container  while  the  scales  are 
again  balanced.  The  weight  required  to  balance  is  the  weight  of 
water  displaced  by  the  specimen,  and  hence  if  in  grams  is  numer- 
ically equal  to  the  volume  of  the  specimen  in  cubic  centimetres. 


THE  MECHANICAL  PROPERTIES  OF  WOOD 


137 


A  diagrammatic  sketch  of  the  arrangement  of  this  apparatus  is 
shown  in  Fig.  51. 

Air-dry  specimens  will  be  dipped  in  water  and  then  wiped  dry 
after  the  first  weighing  and  just  before  being  immersed  for  weighing 


it 


FIG.  51. — Diagram  of  specific  gravity  apparatus,  showing  a  balance  with  con- 
tainer (c)  filled  with  water  in  which  the  test  block  (6)  is  held  submerged  by  a  light 
rod  (a)  which  is  adjustable  vertically  and  provided  with  a  sharp  point  to  be  driven 
into  the  specimen. 

their  displacement.  All  displacement  determinations  will  be  made 
as  quickly  as  possible  in  order  to  minimize  the  absorption  of 
water  by  the  specimen. 


STRENGTH  VALUES   FOR  STRUCTURAL  TIMBERS 
(From  Cir.  189,  U.  S.  Forest  Service) 

The  following  tables  bring  together  in  condensed  form  the 
average  strength  values  resulting  from  a  large  number  of  tests 
made  by  the  Forest  Service  on  the  principal  structural  timbers  of 
the  United  States.  These  results  are  more  completely  discussed 
in  other  publications  of  the  Service,  a  list  of  which  is  given  on 
pages  157-159. 

The  tests  were  made  at  the  laboratories  of  the  U.  S.  Forest 
Service,  in  cooperation  with  the  following  institutions:  Yale  Forest 
School,  Purdue  University,  University  of  California,  University  of 
Oregon,  University  of  Washington,  University  of  Colorado,  and 
University  of  Wisconsin. 

Tables  XVIII  and  XIX  give  the  average  results  obtained  from 
tests  on  green  material,  while  Tables  XX  and  XXI  give  average 
results  from  tests  on  air-seasoned  material.  The  small  specimens, 
which  were  invariably  2"  X  2"  in  cross  section,  were  free  from 
defects  such  as  knots,  checks,  and  cross  grain;  all  other  specimens 
were  representative  of  material  secured  in  the  open  market.  The 
relation  of  stresses  developed  in  different  structural  forms  to  those 
developed  in  the  small  clear  specimens  is  shown  for  each  factor 
in  the  column  headed  "  Ratio  to  2"  X  2"."  Tests  to  determine 
the  mechanical  properties  of  different  species  are  often  confined  to 
small,  clear  specimens.  The  ratios  included  in  the  tables  may  be 
applied  to  such  results  in  order  to  approximate  the  strength  of  the 
species  in  structural  sizes,  and  containing  the  defects  usually 
encountered,  when  tests  on  such  forms  are  not  available. 

A  comparison  of  the  results  of  tests  on  seasoned  material  with 
those  from  tests  on  green  material  shows  that,  without  exception, 
the  strength  of  the  2"  X  2"  specimens  is  increased  by  lowering  the 
moisture  content,  but  that  increase  in  strength  of  other  sizes  is 
much  more  erratic.  Some  specimens,  in  fact,  show  an  apparent 
loss  in  strength  due  to  seasoning.  If  structural  timbers  are 

138 


THE  MECHANICAL  PROPERTIES  OF  WOOD       139 

seasoned  slowly,  in  order  to  avoid  excessive  checking,  there 
should  be  an  increase  in  their  strength.  In  the  light  of  these  facts 
it  is  not  safe  to  base  working  stresses  on  results  secured  from  any 
but  green  material.  For  a  discussion  of  factors  of  safety  and  safe 
working  stresses  for  structural  timbers  see  the  Manual  of  the 
American  Railway  Engineering  Association,  Chicago,  1911.  A 
table  from  that  publication,  giving  working  unit  stresses  for 
structural  timber,  is  reproduced  on  page  144  of  this  book. 


140  THE    MECHANICAL    PROPERTIES    OF    WOOD 

TABLE  XVIII 

BENDING    TESTS    ON    GREEN    MATERIAL 


Cal- 

Sizes 

£ 

F.  S.  at  E.L. 

M.  of  R. 

M.  of  E.          culated 

shear 

Species 

•s 

I 
"8 

| 

CM 

p,  o 

*• 

N 

; 

hi;        S-= 

ft  w     j   ^             £X  o 

.. 

1  « 

1 

1 

! 

l'« 

1- 

t'fi 

°^ 

I'S    5~>  '  I'l 

°N 

lv§ 

a 

a 

| 

" 

i 

£   3 

J* 

£  3 

•J^> 

1  1     •-§_£    1  1 

v§jj 

o 

02 

fc 

£ 

s 

<  m 

<  S 

« 

<  %  \  «    -<s 

fS 

1000  ' 

Inches 

7ns. 

Lbs. 

Lbs. 

/6s.                 Lbs. 

Longleaf  pine.  .  .    .    12  by  12     138 

4    28.6      9  7 

4,099 

0.83 

6,710 

0.74 

1,523     0.99    261 

0.86 

10  by  16    168 

4    26.8     16.7 

4,193 

.85 

6,453 

.71 

1,626     1.05    306 

1.01 

8  by  16    156 

7  !  28.4     14.6 

3,147 

.64 

5,439 

.60 

1,368       .89    390 

1.29 

6  by  16  1  132 

1    40.  3  121.  8 

4,120 

.83 

6,460 

.71 

1,190       .77    378 

1  25 

6  by  10     180 

1    31.  0|    6.2 

3,580 

.  72 

6,500 

.72 

1,412       .92     175 

.58 

6  by    8     180 

2    27.  0      8.2 

3,735 

.75 

5,745 

.63 

1,282  I     .83     121 

.40 

2  by    2      30 

15    33.9     14  1 

4,950 

1.00 

9,070 

1.00 

1,540  1  1.00    303 

1  00 

Douglas  fir  

8  by  16    180 

191     31.5     11  0 

3,968 

.76 

5,983 

.72 

1,517       .95    269 

.81 

5  by    8  i  180 

84    30.1  j  10.8 

3,693 

.71 

5,178 

.63 

1,533  1     .96     172 

.52 

2  by  12 

180 

27    35.7  \  20.3 

3,721 

.71 

5,276 

.64 

1,642     1.03    256 

.77 

2  by  10 

180 

26  i  32.9    21  6 

3,160 

.60 

4,699 

.57 

1,593     1.00     189 

.57 

2  by    8 

180 

29    33.6     17.6 

3,533 

.63 

5,352 

.65 

1,607     1.01     171 

.51 

2  by    2 

24 

568    30.4     11.6 

5,227 

1.00 

8,280 

1.00 

1,597     1.00    333 

1.00 

Douglas   fir    (fire- 

killed)  

8  by  16 

180 

30 

36.8 

10.9 

3,503 

.80 

4,994 

.64 

1,531       .94    330 

1.19 

2  by  12 

180 

32  !  34.  2     17.7 

3,489 

.80 

5,085 

.66 

1,624       .99    247 

.89 

2  by  10  :  180 

32    38.9     18.1 

3,851 

.88 

5,359 

.69 

1,716     1.05    216 

.78 

2  by    8     180 

31     37.0     15  7 

3,403 

.78 

5,305 

.68 

1,676     1  02     169 

.61 

2  by    2      30 

290    33.2     17.2 

4,360 

1.00 

7,752 

1.00 

1,636     1.00    277 

1.00 

Shortleaf  pine  

8  by  16    180 

12    39.5     12  1 

3,185 

.73 

5,407 

.70 

1,438     1.03    362 

1.40 

8  by  14     180 

12  '  45.8     12.7 

3,234 

.74 

5,781 

.75 

1,494     1.07     338 

1.31 

8  by  12 

180 

24    52.2     11.8 

3,265 

.75 

5,503 

.71 

1,480     1.06    277 

1.07 

5  by    8     180 

24    47.8     11  5 

3,519 

.81 

5,732 

.74 

1,4S5     1.06     185 

.72 

2  by    2  i    30 

254    51.7     13.6 

4,350 

1.00 

7,710 

1.00 

1,395     1.00     258 

1.00 

Western  larch  

8  by  16    180 

32    51.0    25.3 

3,276 

.77 

4,632 

.64 

1,272       .97    298 

1.11 

8  by  12     180 

30    50.3     23.2 

3,376 

.79 

5,286 

.73 

1,331       .02    254 

.94 

5  by    8     180 

14,56.0    25.6 

3,528 

.83 

5,331 

.74 

1,432       .09     169 

.63 

2  by    2  i    28 

189    46.2    26.2 

4,274 

1.00 

7,251 

1.00 

1,310       .00    269 

1.00 

Loblolly  pine  

8  by  16     180 

17  |  '-5.8      6.1 

3,094 

.75 

5,394 

69 

1,406       .98    383 

1.44 

5  by  12     180 

94     60.9      5.9 

3,030 

.74 

5,028 

.64 

1,383       .96    221 

.83 

2  by    2      30  ' 

44    70.9      54 

4,100 

1.00 

7,870 

1.00 

1,440       .00    265 

1.00 

Tamarack  

6  by  12     162 

15    57.6     16.6 

2,914 

.75 

4,500 

.66 

1,202       .05    255 

1.11 

4  by  10  i  162 

15    43  .  5     11.4 

2,712 

.70 

4,611 

.68 

1,238       .08    209 

.91 

2  by    2  ;    30 

82    38.8     14.0 

3,875 

1.00 

6,820 

1.00 

1,141        00    229 

1.00 

Western  hemlock.  .      8  by  16     180 

39    42.5     15.6 

3,516  i     .80 

5,296 

.73 

1,445       .01     261 

.92 

2  by    2 

28 

52    51.8 

12.1 

4.406 

1.00 

7,294 

1.00 

1,428       .00    284 

1.00 

Redwood. 

8  by  16 

180 

14     86  5 

19.9 

3,734 

.79 

4,492 

64 

1,016        96    300 

1  21 

6  by  12 

180 

14 

87.3 

3,787 

.80 

4,451 

.'64 

1,068       '.00    224 

^90 

7  by    9 

180 

14 

79.8     16.7 

4,412 

.93 

5,279 

.76 

1,324       .25     19!) 

.80 

3  by  14 

180 

13 

86.1  !  23.7 

3,506 

.74 

4,364 

.62 

947       .89    255 

1.03 

2  by  12 

180 

12 

70.9 

18.6 

3,100 

.65 

3,753 

.54 

1,052       .99     187 

.  75 

2  by  10 

180  I 

13 

55.8 

20.0 

3,285 

.69 

4,079 

.58 

1,107        04     169 

.68 

2  by    8 

180  i 

13 

63.8 

21.5 

2,989 

.63 

4,063 

.58 

1,141       .08     134 

.54 

2  by    2 

28 

157 

75.5     19.1 

4,750 

1.00 

6,980 

1.00 

1,061       .00    248 

1.00 

Norway  pine  

6  by  12     162 

15 

50.3  i  12.5 

2,305 

.82 

3,572 

.69 

987       .03     201 

1.17 

4  by  12  |  162 

18  ,  47.9     14.7 

2,648 

.94 

4,107 

.79 

1,255       .31     238 

1.38 

4  by  10 

162 

16    45.7 

13.3 

2,674 

95 

4,205 

.81 

1.306       .36     198 

1.15 

2  by    2 

30 

133    32.3 

11.4 

2,808 

1.00 

5,173 

1.00 

960       .00     172 

1.00 

Red  spruce  

2  by  10 

144 

14    32.5 

21.9 

2,394 

.66 

3,566 

.60 

1,180       .02  ;  181 

.80 

2  by    2 

26 

60    37.3 

21.3 

3,627 

1.00 

5,900 

1.00 

1,157       .00  :  227 

1.00 

White  spruce  2  by  10 

144 

16    40.7  !    9.3 

2,239 

72 

3,288 

.63 

1,081       .08  .  166 

.83 

2  by    2 

26 

83    58.3  i  10.2 

3,090 

1.00 

5,185 

1.00 

998       .00     199 

1.00 

THE    MECHANICAL    PROPERTIES    OF   WOOD 


141 


TABLE  XIX 

COMPRESSION    AND    SHEAR    TESTS    ON    GREEN    MATERIAL 


Compression  ||  to  grain 

Compression  1  to  grain 

Shear 

1 

1 

1 

T-r 

3 

1 

1 

Species 

1 

a 

**"$ 

1 

SJ 

1 

1 

ri-3 

1 

1 

be 

1 

"8 

•8 

.g.i 

. 

"S  1 

1 

"8 

"8 

^.2 

"8 

"8 

£ 

i 

la 

2  | 

W_^ 

*  ? 

« 

*> 

J 

1 

J  § 

M 

"S 

"I 

® 

a 

o 

x  o* 

o  H 

02  o3 

cS 

.SP      S 

O 

CO     -j, 

a 

55 

i 

1 

1 

C  M 

O 

^'~ 

o 

£ 

n 

3 

*H 

fin 

o 

£ 

1 

00 

1,000 

Inches 

Lbs. 

Ibs. 

Lbs. 

/ncAes 

In. 

L6s. 

L6s. 

Longleaf  pine.  ...    4  by  4      46 

26.3 

3,480 

4,800 

4  by  4 

4 

22 

25.3 

568 

44 

21.8 

973 

2  by  2      14 

34.7 

'.     4,400 

Douglas  fir  6  by  6    515 

30.7    2,780     1,181     3,500 

4  by  8 

ie 

259 

30~3 

570 

531 

29^7 

765 

5  bv  6     170 

30  9    2  720    2  123    3  490 

2  by  2    902 

29.8    3,500     1,925    4,030 

Douglas  fir  (fire- 

killed)  6  by  6     108 

34.8 

2,620     1,801     3,290 

6  by  8 

16 

24    33.7 

368 

77 

35.8 

631 

2  by  2    204 

37.9 

3,430 

Shortleaf  pine  ...    6  by  6      95 

41.2 

2,514     1,565    3,436 

5  by  8 

ie 

'12    37.7 

361 

i79    47.0 

/04 

5  by  8      23 

43.5 

2,241     1,529    3,423 

5  by  8 

14 

12    42.8 

366 

2  by  2  |  281 

51.4 

3,570 

5  by  8 

12 

24 

53.0 

325 

5  by  5 

8      24    47  0 

344 

u  uy   *j 

2  by  2 

2    277    48  5    400 

Western  larch  ...    6  bv  6     107 

49  !i     2,675     1,575    3,510 

6  by  8 

16      22  '  43.  6  i  417 

i79 

40^7 

700 

2  by  2    491 

50.6    3,026  ,  1,545    3,696 

6  by  8 

12       20    40.2    416 

1 

4  by  6 

6 

53    52.8  i  478 



'.'.'.'.  \    '.'.'.'. 

4  by  4 

4 

30    50.4 

472 

Loblolly  pine.  ...    8  by  8      14 

63^4 

1,560 

365    2,i40 

8  by  4 

8 

16    67.2 

392 

121 

83^2 

630 

4  by  8       18 

60.0 

2,430 

691     3,560 

4  by  4 

8 

38    44.6 

546 

2  by  2 

53 

74.0 

3,240 

Tamarack  6  by  7 

4 

49.9 

2,332     1,432    3,032 

'24 

39.2 

668 

4  by  7 

6 

27.7    2,444     1,334    3,360 

2  by  2 

165 

36.8  :    

3,190 

Western  hem- 

lock      6  by  6      82 

46.6    2,905     1,617    3,355 

6  by  4 

6 

30    48.7 

434 

54 

65.7 

630 

2  by  2     131 

55.6    2,938     1,737    3,392 

Redwood  6  by  6  i    34 

83.6 

3,194     1,240    3,882     6  by  8 

16 

'13    86.7 

473 

i48 

84^2 

742 

2  by  2 

143 

72.1 

3,490     1,222  ,  3,980    6  by  6 

12 

14  ;  83.0 

424 

6  by  7 

9 

13    74.7 

477 

.  .  . 

6  by  3 

14 

13 

75.6 

411 

6  by  2 

12 

12 

66.5 

430 

6  by  2 

10 

11 

55.0 

423 

6  by  2 

8 

56.7 

396 

2  by  2 

2 

186 

75.5 

569 

Norway  pine  ....    6  by  7 

"5 

29.0 

1,928 

'905    2,404 

20 

26.7 

589 

4  by  7        8 

28.4 

2,154     1  063  .  2,652 

2  by  2  i  178 

26.8 

2.504     

Red  spruce  2  by  2      58 

35.4 

....     2,750    2  by  2 

'2      43    3L8 

310 

'30    32.0 

758 

White  spruce  2  by  2      84 

61.0 

....     2.370    2  by  2 

2      46    50.4 

270      40    58.0 

651 

142 


THE    MECHANICAL    PROPERTIES    OF   WOOD 


TABLE  XX 

BENDING    TESTS    ON    AIR-SEASONED    MATERIAL 


Cal- 

Sizes 

QJ 

F.S.atE.L. 

M.  of  R. 

M.  of  E. 

culated 

•*§ 

1 

shear 

Species 

J 
"8 

I 

"8 

1 

|| 

; 

K* 

Iq 

frfl 

0} 

&* 

i. 

1 

S3 

-g 

H 

3|, 

8)'^ 

Si, 

§o'~ 

•3; 

t.5 

_0; 

a 

•2 

0 

§> 

2  & 

.2  >i 

2  § 

2  § 

•-  °>> 

S, 

•2^ 

*3 

IU 

j 

>  ^ 

"*3  ,O 

>  ^ 

"c3  "^ 

>  ^ 

"cd  "*"* 

>  ^ 

I-S 

0 

& 

iz; 

ft 

PH 

<J  S" 

« 

«<  S* 

K 

<  i1 

« 

<  E 

« 

1,000 

Inches. 

Ins. 

Lbs. 

Lbs. 

/6s. 

Lbs. 

Longleaf  pine  

8  by  16 

180 

5 

22.2 

16.0 

3,390 

0.50 

4,274 

0.37 

1,747 

1.00 

288 

0.75 

6  by  16 

132 

1 

23.4 

17.1 

3,470 

.51 

6,610 

.57 

1,501 

.86  ;  388 

1.01 

6  by  10 

177 

2 

19.0 

8.8 

4,560 

.68 

7,880 

.68 

1,722 

.99    214 

.56 

4  by  11 

180 

1 

18.4 

23.9 

3,078 

.46 

8,000 

.69 

1,660 

.95 

251 

.66 

6  by    8 

177 

6 

20.0 

13.7 

4,227 

.63 

8,196 

.71 

1,634 

.94 

177 

.46 

2  by    2 

30 

17 

15.9 

13.9 

6,750 

1.00 

11,520;  1.00     1,740 

1.00 

383 

1.00 

Douglas  fir  

8  by  16 

180 

91 

20.8 

13.1 

4,563 

.68 

6,372 

.61 

1,549 

.91     269 

.64 

5  by    8 

180 

30 

14.9 

12.2 

5,065 

.76 

6,777 

.65 

1,853 

1,09 

218 

.52 

2  by    2 

24 

211 

19.0 

16.4 

6,686 

1.00 

10,378 

1.00 

1,695 

1.00 

419 

1.00 

Shortleaf  pine  

8  by  16 

180 

3 

17.0 

12.3 

4,220 

.54 

6,030 

.50 

1,517 

.85  '  398 

.98 

8  by  14 

180 

3 

16.0 

12.3 

4,253 

.55 

5,347 

.44 

1,757 

.98  !  307 

.76 

8  by  12 

180 

7 

16.0 

12.4 

5,051 

.65 

7,331 

.60 

1,803 

1.01 

361 

.89 

5  by    8 

180 

6 

12.2 

22.5 

7,123 

.92 

9,373 

.77 

1,985 

1.11 

301 

.74 

2  by    2 

30 

67 

14.2 

13.7 

7,780 

1.00 

12,120 

1.00 

1,792     1.00 

404 

1.00 

Western  larch  

8  by  16 

180 

23 

18.3 

21.9 

3,343 

.57 

5,440 

.53 

1,409  !     .90 

349 

.96 

8  by  12 

180 

29 

17.8 

23.4 

3,631 

.62 

6,186 

.60 

1,549 

.99 

295 

.81 

5  by    8 

180 

10 

13.6 

27.6 

4,730 

.80 

7,258 

.71 

1,620 

1.04 

221 

.61 

2  by    2 

30 

240 

16.1 

26.8 

5,880 

1.00 

10,254 

1.00 

1,564 

1.00 

364 

1.00 

Loblolly  pine 

8  by  16 

180 

14 

20  5 

7  4 

4,195 

.81 

6,734 

.72 

1,619 

1  10 

462 

1  45 

6  by  16 

126 

4 

20.2 

5.0 

2,432 

.47 

4,295 

.46 

1,324 

^90 

266 

^84 

6  by  10 

174 

3 

21.3 

4.7 

3,100 

.60 

6,167 

.66 

1,449 

.99 

173 

.54 

4  by  12 

174 

4 

19.8 

4.7 

2,713 

.52 

5,745 

.61 

1,249 

.85 

185 

.58 

8  by    8 

180 

9 

22.9 

4.9 

2,903 

.56 

4,557 

.48 

1,136 

.77 

93 

.29 

6  by    7 

144 

2 

21.1 

5.0 

2,990 

.58 

4,968 

.53 

1,286 

.88 

116 

.36 

4  by    8 

132 

8 

19.5 

9.1 

3,384 

.65 

6,194 

.66 

1,200 

.82 

196 

.62 

2  by    2 

30 

123 

17.6 

6.6 

5,170 

1.00 

9,400 

1.00 

1,467     1.00 

318 

1.00 

Tamarack 

6  by  12 

162 

5 

23  0 

15.1 

3,434 

.45 

5,640 

43 

1,330        82 

318 

.75 

4  by  10 

162 

4 

14.4 

4,100 

.54 

5,320 

.41 

1,356       .'84 

252 

2  by    2 

30 

47 

11.3 

16^2 

7,630 

1.00 

13,080 

1.00 

1,620 

1.00 

425 

LOO 

Western  hemlock  .  . 

8  by  16 

180 

44 

17.7 

17.8 

4,398 

.69 

6,420 

.62 

1,737 

1.04 

406 

1.06 

2  by    2 

28 

311 

17.9 

19.4 

6,333 

1.00 

10,369 

1.00 

1,666 

1.00 

382 

1.00 

Redwood  

8  by  16 

180 

6 

26.3 

22.4 

3,797 

.79 

4,428 

.57 

1,107 

.96 

294 

1.05 

6  by  12 

ISO 

6 

16.1 

17.7 

3,175 

.66 

3,353 

.43 

728  !     .64 

167. 

.60 

7  by    9 

180 

6 

15.9 

15.2 

3,280 

.69 

4,002 

.51 

1,104       .96 

147 

.53 

3  by  14 

180 

6 

13.1 

24.4 

5,033 

.64 

291 

1.04 

2  by  12 

180 

5 

13.8 

14.4 

3,928 

".82 

5,336      .68 

l',249     1.09 

260 

.93 

2  by  10 

180 

5 

13.8 

24.8 

3,757 

.79 

4,606i     .59 

1,198     1.05     186 

.67 

2  by    8 

180 

6 

13.7 

20.7 

4,314 

.90 

5,050      .65 

1,313  j  1.15     166 

.60 

2  by    2 

28 

122 

15.2 

18.8 

4,777 

1.00 

7,798   1.00 

1,146 

1.00    279 

1.00 

Norway  pine  

6  by  12 

162 

5 

16.7 

8.1 

2,968 

.56 

5,204 

.61 

1,123 

.97 

286 

1.02 

4  by  10 

162 

5 

13.7 

12.0 

5,170 

.98 

6,904 

.82 

1,712 

1.48 

317 

1.13 

2  by    2 

30 

60 

14.9     11.2 

5,280    1.00 

8,470   1.00 

1,158     1.00    281     1.00 

THE    MECHANICAL    PROPERTIES    OF    WOOD 


143 


TABLE  XXI 

COMPRESSION    AND    SHEAR    TESTS    ON    AIR-SEASONED    MATERIAL 


Compression  II  to  grain 

Compression  1  to  grain 

Shear 

I 

I 

2 

£ 

1 

£ 

Species 

§ 

« 

1 

J 

3 
i1 

I 

J 

.1 

1 

.9 

J 

| 

H"" 

K 

p'g 

I 

w| 

i 

f"^"i 

0 

15 

-3 

-M.S 

-^  c« 

8 

•3 

•3 

"3 

•3 

•8 

1 

1 

|| 

*1 

*l 

£ 

-a 

J 

•(i! 

Jfi 

1 

«  i 

m 

1 

& 

o 

s'~ 

^  a 
o 

1 

•3 

w 

1 

I 

6* 

1 

1 

J* 

1,000 

Inches 

Lbs. 

Ibs. 

Lbs. 

Inches 

In. 

Lbs. 

Lbs. 

Longleaf  pine.  .  .  . 
Douglas  fir  

4  by  5 
6  by  6 
2  by  2 

46 
259 
247 

26.3 
20.3 

18.7 

3,480 
3,271 
3,842 

l',038 
1,084 

4,800 
4,258 
5,002 

by  5  ;    4 
by  8     16 
by  8     10 

22 
44 
32 

25.1 

20.8 
18.1 

572 
732 

584 

52   20.2 
465   22.1 

984 
822 

by  4      8 

51 

20.2     638 

by  4      6 

49    24.0    613 

'..  I... 

by  4 

4 

29    24.8     603 

| 

Shortleaf  pine...  . 

6  by  6 

2  by  2 

'29 
57 

15.7 
14.2 

4,670 

1,951 

6,030 
6,380 

8  by  5     16 
8  by  5     14 

4     17.8    725 
3     16.3     757 

85  1  ;  ;  ;  ; 

1,135 

8  by  5     12        5     15.1     730 

5  by  5      8        6     13.0    918 

2  by  2      2      57     13.9    926 

Western  larch  .  .  . 

6  by  6 

112 

16^0      .... 

5,445 

8  by  6     16      17     18.8    491 

i93  is^o 

905 

4  by  4 

81 

14.7 

6,161 

8  by  6     12 

18  1  17.6    526 

2  by  2 

270 

14.8 

5,934 

5  by  4      8 

22 

13.3  i  735 

'.'.  \','.'. 

Loblolly  pine.  .  .  . 

6  by  6 
5  by  5 

23 
10 

22!4 

3357 
2.217 

1,693 

545 

5,005 
2.950 

8  by  5     16 

8  by  5      8 

12     19.8     602 
7  '  22.9     679 

156   11.3 

! 

i.iis 

4  by  8 

8 

19.4    SiOlO 

633    3,920 

4  by  5 

8 

8    19.5     715 

2  by  2 

69 

5,547 

Tamarack  

6  by  7 

3 

15.  7    2,257 

l',042 

3,323 

2  by  2 

'2 

'57 

ie^ 

697 

'60   14  !o 

879 

4  by  7 

3  i  13.6  ,  3,780 

1,301  i  4,823 

4  by  4 

57     14^9    3!386 

U53    4^346 

2  by  2 

66     14.6      . 

4.790 

'.'.     .... 

West,  hemlock.. 

6  by  6 

102 

18.6 

4,840 

2,140    5,814: 

7  by  6     15 

25     18.2    514 

isi 

17^7 

924 

2  by  2 

463 

17.0 

4,560 

1,923    5,403 

6  by  6      6 

26     16.8  :  431 

4  by  4      4 

6     15.9    488 

Redwood  

6  by  6 

'is 

16^9 

4,276 

8  by  6     16 

5    25.4    548 

'95 

12^4 

67l 

2  by  2 

115 

14^6 

5.119 

6  by  6     12 

6    14.7  ;  610 

7  by  6      9 

5  i  14.8    500 

3  by  6     14 

2     12.6    470 

'.  '.  '. 

2  by  6     12 

2     16.2    498 

2  by  6     10 

4     14.3    511 

! 

2  by  6 

8 

2     13.2    429 

2  by  2 

2 

145     13.8    564 

Norway  pine  .... 

6  by  7 

4 

15.2    2,670 

1,182    4,2l2 

2  by  2 

2 

36     10.0    924 

44   ii.9 

1,145 

4  by  7 

2     22  2     3.275     1.724     4.575 

4  by  4      55     16^6    3^048     U67    4^217 

2  by  2      44 

11.2      7,550     

NOTE. — Following  is  an  explanation  of  the  abbreviations  used  in  the  foregoing  tables: 
F.  S.  at  E.  L.  =  Fiber  stress  at  elastic  limit. 
M.  of  E.  =  Modulus  of  elasticity. 
M.  of  R.  =  Modulus  of  rupture. 
Cr.  str.  at  E.  L.  =  Crushing  strength  at  elastic  limit. 
Cr.  str.  at  max.  Id.  =  Crushing  strength  at  maximum  load. 


144 


THE    MECHANICAL    PROPERTIES    OF    WOOD 


TABLE 

*  WORKING   UNIT-STRESSES   FOR   STRUCTURAL  TIMBER  f 
EXPRESSED   IN   POUNDS   PER  SQUARE   INCH 

(From  Manual  of  the  American  Railway  Engineering  Assn.,  1911,  p.  153) 

NOTE. — The  working  unit-stresses  given  in  this  table  are  intended  for  railroad  bridges  and  trestles.  For 
highway  bridges  and  trestles  the  unit-stresses  may  be  increased  twenty-five  (25)  per  cent.  For  buildings  and 
similar  structures,  in  which  the  timber  is  protected  from  the  weather  and  practically  free  from  impact,  the 
unit-stresses  may  be  increased  fifty  (50)  per  cent.  To  compute  the  deflection  of  a  beam  under  long-continued 
loading  instead  of  that  when  the  load  is  first  applied,  only  fifty  (50)  per  c?.ut  of  the  corresponding  modulus 
of  elasticity  given  in  the  table  is  to  be  employed. 


-nu}  •:  jo  q;§u9j 


J9AO  suunjoo 
2tio|  m  ssaa^s  Sni 
-JJJOA' 


joA\  'sureip 
cj  japin  suranpo'jOjj 


' 


SS9IJS 


llel  t 
rain 


93BJ9AV 


s  g  g  g  g 


g    8    8 


o  |  o  I 

g  § 


I  o  I  =  1  o 

g  I  1 


1  I 


S|8JS_ 

2        S 


th  in  in 
t  side  in 


*  Adopted,  Vol.  1909,  pp.  537,  564,  609-611. 
f  Green  timber  in  exposed  work. 


BIBLIOGRAPHY 

Part      I :   Some  general  works  on  mechanics,  materials  of  construc- 
tion, and  testing  of  materials. 

Part    II:   Publications  and  articles  on  the  mechanical  properties 
of  wood,  and  timber  testing. 

Part  III :   Publications  of  the  U.  S.  Government  on  the  mechanical 
properties  of  wood,  and  timber  testing. 


I.    SOME   GENERAL   WORKS   ON   MECHANICS,  MATE- 
RIALS OF  CONSTRUCTION,  AND    TESTING    OF 
MATERIALS 

ALLAN,  WILLIAM:    Strength    of   beams    under   transverse  loads. 

New  York,  1893. 
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London,  1902. 

BARLOW,  PETER:    Strength  of  materials,  1st  ed.  1817;  rev.  1867. 
BURR,  WILLIAM  H. :  The  elasticity  and  resistance  of  the  materials 

of  engineering.     New  York,  1911. 

CHURCH,  IRVING  P.:  Mechanics  of  engineering.    New  York,  1911. 
HATFIELD,  R.  G.:  Theory  of  transverse  strain.    1877. 
HATT,  W.  K.,  and  SCOFIELD,  H.  H. :  Laboratory  manual  of  test- 
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JAMESON,    J.    M.:    Exercises   in   mechanics.       (Wiley   technical 

series.)     New  York,  1913. 
JAMIESON,  ANDREW:   Strength  of  materials.     (Applied  mechanics 

and  mechanical  engineering,  Vol.  II.)     London,  1911. 
JOHNSON,  J.  B. :  The  materials  of  construction.    New  York,  1910. 
KENT,  WILLIAM  :  The  strength  of  materials.    New  York,  1890. 
KOTTCAMP,  J.  P.:    Exercises  for  the  applied  mechanics   labora- 
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LANZA,  GAETANO:    Applied  mechanics.     New  York,  1901. 
MERRIMAN,  MANSFIELD:    Mechanics  of  materials.      New  York, 

1912. 

MURDOCK,  H.  E.:   Strength  of  materials.    New  York,  1911. 
RANKINE,  WILLIAM  J.   M.:    A  manual    of  applied    mechanics. 

London,  1901. 
THIL,  A. :    Conclusion  de  1'etude  presentee  a  la  Commission  des 

methodes  d'essai  des  materiaux  de  construction.    Paris,  1900. 
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engineering:  stone,  timber,  fuel,  lubricants,  etc.     (Materials  of 

engineering,  Part  I.)    New  York,  1899. 
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London,  1899. 
WATERBURY,  L.  A.:    Laboratory  manual   for   testing   materials 

of  construction.     New  York,  1912. 
WOOD,  DEVOLSON:    A   treatise   on  the   resistance  of   materials. 

New  York,  1897. 

147 


148  THE   MECHANICAL   PROPERTIES    OF   WOOD 

II.    PUBLICATIONS  AND  ARTICLES  ON  THE  MECHAN- 
ICAL   PROPERTIES    OF    WOOD,    AND    TIMBER 
TESTING 

ABBOT,  ARTHUR  V.:  Testing  machines,  their  history,  construc- 
tion and  use.  Van  Nostrand's  Eng.  Mag.,  Vol.  XXX,  1884,  pp. 
204-214;  325-344;  382-397;  477-490. 

ADAMS,  E.  E. :  Tests  to  determine  the  strength  of  bolted  timber 
joints.  Cal.  Jour,  of  Technology,  Sept.,  1904. 

ALVAREZ,  ARTHUR  C.:  The  strength  of  long  seasoned  Douglas 
fir  and  redwood.  Univ.  of  Cal.  Pub.  in  Eng.,  Vol.  I,  No.  2, 
Berkeley,  1913,  pp.  11-20. 

BARLOW,  PETER  :  An  essay  on  the  strength  and  stress  of  timber. 
London,  1817;  3d  ed.,  1826. 

— :  Experiments  on  the  strength  of  different  kinds 
of  wood  made  in  the  carriage  department,  Royal  Arsenal, 
Woolwich.  Jour.  Franklin  Inst.,  Vol.  X,  1832,  pp.  49-52. 
Reprinted  from  Philosophical  Mag.  and  Annals  of  Philos.,  No. 
63,  Mch.,  1832. 

BATES,  ONWARD:  Pine  stringers  and  floorbeams  for  bridges. 
Trans.  Am.  Soc.  C.  E.,  Vol.  XXIII. 

BAUSCHINGER,  JOHANN  :  Untersuchungen  liber  die  Elasticitat  und 
Festigkeit  von  Fichten-  und  Kiefernbauholzern.  Mitt.  a.  d. 
mech.-tech.  Laboratorium  d.  k.  techn.  Hochschule  in  Miinchen, 
9.  Hft.,  Munchen,  1883. 

-  :  Verhandlungen  der  Miinchener  Conferenz  und 
der  von  ihr  gewahlten  standigen  Commission  zur  Vereinbarung 
einheitlicher  Priifungsmethoden  fur  Bau-  und  Constructions- 
material.  Ibid.,  14.  Hft.,  1886. 

:  Untersuchungen  liber  die  Elasticitat  und  Festigkeit 


verschiedener  Nadelholzer.     Ibid.,  16.  Hft.,  1887. 

BEARE,  T.  HUDSON:  Timber:  its  strength  and  how  to  test  it. 
Engineering,  London,  Dec.  9,  1904. 

BEAUVERIE,  J. :  Le  bois.     I.    Paris,  1905,  pp.  105-185. 

— :   Les  bois  industriels.    Paris,  1910,  pp.  55-77. 

Bending  tests  with  wood,  executed  at  the  Danish  State  Testing 
Laboratory,  Copenhagen.  Proc.  Int.  Assn.  Test.  Mat.,  1912, 
XXIII2,  pp.  17.  See  also  Eng.  Record,  Vol.  LXVI,  1912,  p.  269. 

BERG,  WALTER  G.:  Berg's  complete  timber  test  record.  Chi- 
cago, 1899.  Reprint  from  Am.  Ry.  Bridges  and  Buildings. 

BOULGER,  G.  S.:   Wood.    London,  1908,  pp.  112-121. 

BOUNICEAU, — :  Note  et  experiences  sur  la  torsion  des  bois.  [N.p., 
n.d.] 

BOVEY,  HENRY  T, :  Results  of  experiments  at  McGill  University, 


THE  MECHANICAL  PROPERTIES  OF  WOOD       149 

Montreal,  on  the  strength  of  Canadian  Douglas  fir,  red  pine, 
white  pine,  and  spruce.  Trans.  Can.  Soc.  C.  E.,  Vol.  IX,  Part  I, 
1895,  pp.  69-236. 

BREUIL,  M.  PIERRE:  Contribution  to  the  discussion  on  the  test- 
ing of  wood.  Proc.  Int.  Assn.  Test.  Mat.,  1906,  Disc,  le,  pp.  2. 

BROWN,  T.  S.:  An  Account  of  some  experiments  made  by  order 
of  Col.  Totten,  at  Fort  Adams,  Newport,  R.  I.,  to  ascertain  the 
relative  stiffness  and  strength  of  the  following  kinds  of  timber, 
viz.:  white  pine  (Pinus  strobus),  spruce  (Abies  nigra),  and 
southern  pine  (Pinus  australis),  also  called  long-leaved  pine. 
Jour.  Franklin  Inst.,  Vol.  VII  (n.  s.),  1831,  pp.  230-238. 

BUCHANAN,  C.  P.:  Some  tests  of  old  timber.  Eng.  News,  Vol. 
LXIV,  No.  23,  1910,  p.  67. 

BUSGEN,  M.:  Zur  Bestimmung  der  Holzharten.  Zeitschrift  f. 
Forst-  und  Jagdwesen.  Berlin,  1904,  pp.  543-562. 

CHEVANDIER,  E.,  et  WERTHEIM,  G.:  Memoire  sur  les  proprietes 
mecaniques  du  bois.  Paris,  1846. 

CIESLAR,  A. :  Studien  iiber  die  Qualitat  rasch  erwachsenen  Fichten- 
holzes.  Centralblatt  f.  d.  ges.  Forstwesen,  Wien,  1902,  pp.  337- 
403. 

CLINE,  MCGARVEY:  Forest  Service  investigations  of  American 
woods  with  special  reference  to  investigations  of  mechanical 
properties.  Proc.  Int.  Assn.  Test.  Mat.,  1912,  XXIII5,  pp.  17. 
— :  Forest  Service  tests  to  determine  the  influence  of 
different  methods  and  rates  of  loading  on  the  strength  and 
stiffness  of  timber.  Proc.  Am.  Soc.  Test.  Mat.,  Vol.  VIII,  1908, 
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:    The  Forest  Products  Laboratory:  its  purpose  and 


work.    Proc.  Am.  Soc.  Test.  Mat.,  Vol.  X,  1910,  pp.  477-489. 

:    Specifications  and    grading   rules  for    Douglas    fir 


timber:  an  analysis  of  Forest  Service  tests  on  structural  timbers. 
Proc.  Am.  Soc.  Test.  Mat.,  Vol.  XI,  1911,  pp.  744-766. 

Comparative  strength  and  resistance  of  various  tie  timbers. 
Elec.  Traction  Weekly,  Chicago,  June  15,  1912. 

DAY,  FRANK  M.:  Microscopic  examination  of  timber  with  re- 
gard to  its  strength.  1883,  pp.  6. 

DEWELL,  H.  D.:  Tests  of  some  joints  used  in  heavy  timber 
framing.  Eng.  News,  Mch.  19,  1914,  pp.  594-598;  e\  seq. 

DORR,  KARL:  Die  Festigkeit  von  Fichten-  und  Kiefernholz. 
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schrift d.  ver.  deutsch.  Ing.,  Bd.  54,  Nr.  36,  1910,  p.  1503. 

DUPIN,  CHARLES:  Experiences  sur  la  flexibilite,  la  force,  et  Felas- 
ticite  des  bois.  Jour,  de  1'Ecole  Polytechnique,  Vol.  X,  1815. 

DUPONT,  ADOLPHE,  et  BOUQUET  DE  LA  GRYE:  Les  bois  indigenes 
et  etrangers.  Paris,  1875,  pp.  273-352. 


150  THE    MECHANICAL    PROPERTIES    OF   WOOD 

ESTRADA,  ESTEBAN  DUQUE  :  On  the  strength  and  other  properties 
of  Cuban  woods.  Van  Nostrand's  Eng.  Mag.,  Vol.  XXIX, 
1883,  pp.  417-426;  443-449. 

EVERETT,  W.  H.:  Memorandum  on  mechanical  tests  of  some 
Indian  timbers.  Govt.  Bui.  No.  6  (o.s.),  Calcutta. 

EXNER,  WILHELM  FRANZ  :  Die  mechanische  Technologic  des 
Holzes.  Wien,  1871.  (A  translation  and  revision  of  Chevandier 
and  Wertheim's  Memoire  sur  les  proprietes  mecaniques  du  bois.) 
— :  Die  technischen  Eigenschaften  der  Holzer.  Lorey's 
Handbuch  der  Forstwissenschaft,  II.  Bd.,  6.  Kap.,  Tubingen, 
1903. 

FERNOW,  B.  E.:  Scientific  timber  testing.  Digest  of  Physical 
Tests,  Vol.  I,  No.  2,  1896,  pp.  87-95. 

FOWKE,  FRANCIS:  Experiments  on  British  colonial  and  other 
woods.  1867. 

GARDNER,  ROLAND:  I.  Mechanical  tests,  properties,  and  uses  of 
thirty  Philippine  woods.  II.  Philippine  sawmills,  lumber 
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THE  MECHANICAL  PROPERTIES  OF  WOOD       151 

effect  of  moisture  on  strength  and  stiffness  of  timber,  together 
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•:    Abstract  of  report  on  the  present   status  of  timber 


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152  THE    MECHANICAL    PROPERTIES    OF   WOOD 

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154  THE    MECHANICAL    PROPERTIES    OF    WOOD 

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•:   Experiments  on  the  strength  of  yellow  pine.  Ibid., 


Vol.  LXXIX,  1880,  pp.  157-163. 

:   Influence  of  time  on  bending  strength  and  elasticity. 


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Vol.  LXXI. 

— :    On  the  effect  of  prolonged  stress  upon  the  strength 


156  THE    MECHANICAL    PROPERTIES    OF   WOOD 

and   elasticity    of    pine    timber.      Jour.    Franklin    Inst.,    Vol. 

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Tlie  theory  of  impact  and  its  application  to  test- 


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Some  results  of  dead  load  bending  tests  of  timber 


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:    Weerstand  van  Hout   loodrecht   op  de  Vezelrichting. 

Ibid.,  May,  1911. 

Buigrastheid  van  Hout.    Ibid.,  May  31,  1913. 


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Untersuchungen  liber  den  Einfluss  des  Blauwerdens  auf  die  Fes- 
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Verfahren  zur  Prufung  v.  Metallen  und  Legierungen,  von  hydrau- 
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WARREN,  W.  H. :  Australian  timbers.     Sydney,  1892. 

— :    The  strength,  elasticity,  and  other  properties  of  New 
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The    strength,   elasticity,   and   other   properties   of 


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The  properties  of  New  South  Wales  hardwood  timbers. 


Builder,  London,  Nov.  1,  1912. 

The     hardwood    timbers    of    New    South    Wales, 


Australia.    Jour.  Soc.  of  Arts,  London,  Dec.  6,  1912. 


THE  MECHANICAL  PROPERTIES  OF  WOOD       157 

WELLINGTON,    A.    M.:     Experiments    on    impregnated    timber. 

Railroad  Gazette,  1880. 
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Eng.  News,  Vol.  XXXIII,  Mch.  14,  1895. 


III.   PUBLICATIONS  OF  THE  U.   S.   GOVERNMENT  ON 

THE  MECHANICAL  PROPERTIES  OF  WOOD, 

AND  TIMBER  TESTING 

MISCELLANEOUS 

House  Misc.  Doc.  42,  pt.  9,  47th  Cong.,  2d  sess.,  1884.  (Vol.  IX, 
Tenth  Census  report.)  Report  on  the  forests  of  North  America 
(exclusive  of  Mexico).  Part  II,  The  Woods  of  the  United  States. 

House  Report  No.  1442,  53d  Cong.,  2d  sess.  Investigations  and 
tests  of  American  timber.  1894,  pp.  4. 

War  Dept.  Doc.  1.  Resolutions  of  the  conventions  held  at  Mu- 
nich, Dresden,  Berlin,  and  Vienna,  for  the  purpose  of  adopting 
uniform  methods  for  testing  construction  materials  with  re- 
gard to  their  mechanical  properties.  By  J.  Bauschinger. 
Translated  by  O.  M.  Carter  and  E.  A.  Gieseler.  1896,  pp.  44. 

War  Dept.  Doc.  11.  On  tests  of  construction  materials.  Trans- 
lations from  the  French  and  from  the  German.  By  0.  M.  Carter 
and  E.  A.  Gieseler.  1896,  pp.  84. 

House  Doc.  No.  181,  55th  Cong.,  3d  sess.  Report  upon  the  for- 
estry investigations  of  the  U.  S.  Department  of  Agriculture, 
1877-1898.  By  B.  E.  Fernow,  1899,  pp.  401.  Contains  chapter 
on  The  work  in  timber  physics  in  the  Division  of  Forestry,  by 
Filibert  Roth,  pp.  330-395. 

FOREST    SERVICE 

Cir.     7 — The  Government  timber  tests  [189-],  pp.  4. 

Cir.  8— Strength  of  " boxed"  or  " turpentine"  timber.  1892, 
pp.  4. 

Bui.  6 — Timber  Physics.  Pt.  I.  Preliminary  report.  1.  Need 
of  the  investigation.  2.  Scope  and  historical  development  of 
the  science  of  "  timber  physics."  3.  Organization  and  methods 
of  timber  examinations  in  the  Division  of  Forestry.  By  B.  E. 
Fernow,  1892,  pp.  57. 

Unnumbered  Cir. — Instructions  for  the  collection  of  test  pieces 
of  pines  for  timber  investigations  [1893],  pp.  4. 


158  THE    MECHANICAL    PROPERTIES    OF   WOOD 

Cir.     9 — Effect  of  turpentine  gathering  on  the  timber  of  long- 
leaf  pine.     By  B.  E.  Fernow  [1893],  p.  1. 
Bui.    8 — Timber  physics.      Pt.  II.    Progress  report.      Results  of 

investigations  on  fongleaf  pine.     1893,  pp.  92. 
Bui.  10 — Timber:  an  elementary  discussion  of  the  characteristics 

and  properties  of  wood.    By  Filibert  Roth.    1895,  pp.  88. 
Bui.   12 — Economical   designing   of   timber   trestle  bridges.     By 

A.  L.  Johnson,  1896,  pp.  57. 
Cir.  12 — Southern    pine,   mechanical    and    physical    properties. 

1896,  pp.  12. 
Cir.  15 — Summary  of  mechanical  tests  on  thirty-two  species  of 

American  woods.    1897,  pp.  12. 
Cir.  18 — Progress  in  timber  physics.     1898,  pp.  20. 
Cir.  19 — Progress    in   timber  physics:    Bald  cypress  (Taxodium 

distichum).    By  Filibert  Roth,  1898,  pp.  24. 
Y.  B.  Extr.  288 — Tests  on  the  physical  properties  of  woods.    By 

F.  E.  Olmstead,  1902,  pp.  533-538. 
Unnumbered  Cir.— Timber  tests.     [1903],  pp.  15. 
Unnumbered  Cir. — Timber  preservation  and  timber    testing  at 

the  Louisiana  Purchase  Exposition.     1904,  pp.  6. 
Cir.  32— Progress   report   on  the   strength  of   structural  timber. 

By  W.  K.  Hatt,  1904,  pp.  28. 

Bui.  58 — The  red  gum.    By  Alfred  Chittenden.    Includes  a  dis- 
cussion of   The  mechanical  properties   of  red   gum  wood,  by 

W.  K.  Hatt.    1905,  pp.  56. 
Cir.  38 — Instructions  to   engineers  of  timber  tests.     By  W.  K. 

Hatt,  1906,  pp.  55.    Revised  edition,  1909,  pp.  56. 
Cir.  39 — Experiments  on  the  strength  of   treated  timber.      By 

W.  K.  Hatt,  1906,  pp.  31.    Revised  edition,  1908. 
Bui.   70 — Effect   of  moisture  upon  the  strength  and  stiffness  of 

wood.    By  H.  D.  Tiemann,  1906,  pp.  144.  ^ 
Cir.  46 — Holding  force  of  railroad  spikes  in  wooden  ties.    By 

W.  K.  Hatt,  1906,  pp.  7. 
Cir.  47 — Strength  of  packing  boxes  of  various  woods.     By  W.  K. 

Hatt,  1906,  pp.  7. 
Cir.  108 — The  strength  of  wood  as  influenced  by  moisture.    By 

H.  D.  Tiemann,  1907,  pp.  42. 
Cir.  115 — Second  progress  report  on  the   strength   of   structural 

timber.     By  W.  K.  Hatt,  1907,  pp.  39. 
Cir.  142 — Tests  of  vehicle  and  implement  woods.    By  H.  B.  Hol- 

royd  and  H.  S.  Betts,  1908,  pp.  29. 

Cir.  146 — Experiments  with  railway  cross-ties.     By  H.  B.  East- 
man, 1908,  pp.  32. 
Cir.  179 — Utilization  of  California  eucalypts.    By  H.  S.  Betts  and 

C.  Stowell  Smith,  1910,  pp.  30. 


THE  MECHANICAL  PROPERTIES  OF  WOOD       159 

Bul.  75 — California  tanbark  oak.  Part  II,  Utilization  of  the 
wood  of  tanbark  oak,  by  H.  S.  Betts,  1911,  pp.  24-32. 

Bul.  88 — Properties  and  uses  of  Douglas  fir.  By  McGarvey 
Cline  and  J.  B.  Knapp,  1911,  pp.  75. 

Cir.  189 — Strength  values  for  structural  timbers.  By  McGarvey 
Cline,  1912,  pp.  8. 

Cir.  193 — Mechanical  properties  of  redwood.  By  A.  L.  Heim, 
1912,  pp.  32. 

Bul.  108 — Tests  of  structural  timbers.  By  McGarvey  Cline  and 
A.  L.  Heim,  1912,  pp.  1231. 

Bul.  112 — Fire-killed  Douglas  fir:  a  study  of  its  rate  of  deterior- 
ation, usability,  and  strength.  By  J.  B.  Knapp,  1912,  pp.  18. 

Bul.  115 — Mechanical  properties  of  western  hemlock.  By  0.  P. 
M.  Goss,  1913,  pp.  45. 

Bul.  122 — Mechanical  properties  of  western  larch.  By  O.  P.  M. 
Goss,  1913,  pp.  45. 

Cir.  213 — Mechanical  properties  of  woods  grown  in  the  United 
States.  1913,  pp.  4. 

Cir.  214 — Tests  of  packing  boxes  of  various  forms.  By  John  A. 
Newlin,  1913,  pp.  23. 

Review  Forest  Service  Investigations.  1913.  [Outline  of  investi- 
gations.] Vol.  I,  pp.  17-21.  A  microscopic  study  of  the  me- 
chanical failure  of  wood,  by  Warren  D.  Brush.  Vol.  II,  pp.  33- 
38. 

Bul.  67,  U.  S.  D.  A.— Tests  of  Rocky  Mountain  woods  for  tele- 
phone poles.  By  Norman  deW.  Betts  and  A.  L.  Heim,  1914, 
pp.  28. 

Bul.  77,  U.  S.  D.  A. — Rocky  Mountain  mine  timbers.  By  Nor- 
man deW.  Betts,  1914,  pp.  34. 

Bul.  86,  U.  S.  D.  A.— Tests  of  wooden  barrels.  By  J.  A.  Newlin, 
1914,  pp.  12. 

REPORTS   OF   TESTS   ON   THE   STRENGTH   OF  STRUCTURAL  MATERIAL, 
MADE   AT   THE    WATERTOWN    ARSENAL,    MASS. 

House  Ex.  Doc.  No.  12,  47th  Cong.,  1st  sess.,  1882.  Strength  of 
wood  grown  on  the  Pacific  slope,  pp.  19-93. 

Senate  Ex.  Doc.  No.  1,  47th  Cong.,  2d  sess.,  1883.  Resistance  of 
white  and  yellow  pines  to  forces  of  compression  in  the  direction 
of  the  fibers,  as  used  for  columns,  or  posts,  pp.  239-395. 

Senate  Ex.  Doc.  No.  5,  48th  Cong.,  1st  sess.,  1884.  Tests  of 
California  laurel  wood  by  compression,  indentation,  shearing, 
transverse  tension,  pp.  223-236.  Tests  of  North  American 
woods  (under  supervision  of  Prof.  C.  S.  Sargent  in  charge  of  the 
forestry  division  of  the  Tenth  Census),  with  16  photographs  of 
fractures  of  American  woods,  pp.  237-347. 


160  THE    MECHANICAL    PROPERTIES    OF   WOOD 

Senate  Ex.  Doc.  No.  35,  49th  Cong.,  1st  sess.,  1885.  Adhesion  of 
nails,  spikes,  and  screws  in  various  woods.  Experiments  on 
the  resistance  of  cut  nails,  wire  nails  (steel),  wood  screws,  lag 
screws  in  white  pine,  yellow  pine,  chestnut,  white  oak,  and  lau- 
rel, pp.  448-471. 

House  Ex.  Doc.  No.  14,  51st  Cong.,  1st  sess.,  1890.  Adhesion  of 
spikes  and  bolts  in  railroad  ties,  pp.  595-617. 

House  Ex.  Doc.  No.  161,  52d  Cong.,  1st  sess.,  1892.  Adhesion  of 
nails  in  wood,  pp.  744-745. 

House  Ex.  Doc.  No.  92,  53d  Cong.,  3d  sess.,  1895.  Woods- 
compression  tests  (endwise  compression),  pp.  471-476. 

House  Doc.  No.  54,  54th  Cong.,  1st  sess.,  1896.  Compression 
tests  on  Douglas  fir  wood,  pp.  536-563.  Expansion  and 
contraction  of  oak  and  pine  wood,  pp.  567-574. 

House  Doc.  No.  164,  55th  Cong.,  2d  sess.,  1898.  Compression 
tests  of  timber  posts,  pp.  405-411.  New  posts  of  yellow  pine 
and  spruce,  pp.  413-450;  Old  yellow  pine  posts  from  Boston 
Fire  Brick  Co.  building,  No.  394  Federal  St.,  Boston,  Mass., 
pp.  451-473. 

House  Doc.  No.  143,  55th  Cong.,  3d  sess.,  1899.  Fire-proofed 
wood  (endwise  and  transverse  tests),  pp.  676-681. 

House  Doc.  No.  190,  56th  Cong.,  2d  sess.,  1901.  Cypress  wood 
for  United  States  Engineer  Corps;  compression  and  transverse 
tests,  pp.  1121-1126.  Old  white  pine  and  red  oak  from  roof 
trusses  of  Old  South  Church,  Boston,  Mass.,  pp.  1127-1130. 
Compression  of  rubber,  balata,  and  wood  buffers,  pp.  1149-1158. 

House  Doc.  No.  335,  57th  Cong.,  2d  sess.,  1903.  Douglas  fir 
and  white  oak  woods.  Transverse  and  shearing  tests;  also 
observations  on  heat  conductivity  of  sticks  over  wood  fires  and 
a  stick  exposed  to  low  temperature.  Expansion  crosswise  the 
grain  of  wood  after  submersion,  pp.  519-561.  Adhesion  of 
lag  screws  and  bolts  in  wood,  pp.  563-578. 


INDEX 


PAGE 

ABRASION 39,  114-117 

Annual  rings 44 

Apparatus,  testing,  94,  99,  102,  104, 

107,    110-111,    114,    117,    118, 

121,  122,  132,  133,  136 

Arborvitae,  6,  9,   13,   16,  20,  27,  32, 

42,57 

Ash 15,  22,  44,  48,  51,  66,  78 

black 20,  27,  32,  40,  42,  56 

white,  9,  13,  16,  20,  27,  32,  40, 
42,  45,  56 

Aspen,  largetooth 13 

Axis,  neutral 23 

BASSWOOD,  9,  13,  16,  20,  27,  32,  40 
42,  44,  56 

Beams 15,  24-37,  92,  94 

cantilever 24 

continuous 24 

simple 24 

Beech,  9,  13,  16,  20,  22,  27,  32,  40, 
42,  51,  57 

Bending  large  beams 94-99 

small  beams 99-102,  132 

strength 2,  22-37,  26,  75 

Bibliography 145-160 

Birch 22,44 

yellow,  9,  13,  16,  20,  27,  32,  40, 
42,57 

Bird-peck 59,  72 

Black  check 59 

Boiling,  effect  of 6,  85 

Bow,  flexure  of  a 4 

Boxheart 54,  82 

Brash 6 

Breaking  strength  of  beams ....      15 

Brittleness 6,  34,  37,  38 

Buckeye 13,  44 

Buckling  of  fibres 15,  77 

Butternut .  .  13 


PAGE 

CANTILEVER 24 

Calibration  of  testing  machines,  92, 1 12 

Case-hardening 80 

Catalpa 44 

Cedar,  Central  American 22 

incense 13,  16,  20,  27,  57 

red 144 

white 22 

Checking 54,  61,  74,  75-84 

Chelura 67 

Cherry,  black 13,  22 

Chestnut,  15,  22,  44,  49,  50,  51,  66,  78 

Cleavability 2,  41 

Cleavage 41,  118,  133 

Coefficient  of  elasticity  (see  Mod- 
ulus of  elasticity) 

expansion 84 

Cold,  effect  of 86 

Color 50,  58-59 

Column,  long 12,  14,  144 

short 15,  102-104 

Compression  across  the  grain, 

94,  104-107,  133 

endwise 12,  92,  94,  102,  132 

failure 34,  104 

parallel  to  grain  (see  C.  end- 
wise) 

perpendicular  to  grain  (see 
C.  right  angles  to  grain) 
right  angles  to  grain ....  94,    133 

Compressive  strength 1,  9,  23 

Compressometer 103 

Coniferous  wood 44 

Cottonwood 44,  55 

Creosote,  effect  of 87 

Cross-arms,  testing 124 

Cross  grain 8,  59-61 

Cross-grained  tension  failure ....     34 

Crushing  strength 2,  9,  54,  75 

formula  for .  .  .104 


101 


162 


INDEX 


PAGE 

Cucumber  tree 13 

Cup  shake 64 

Cypress,  bald,  9,  13,  16,  20,  27,  32,  40, 
42,  57,  68,  144 

DEAD  LOAD  (see  Load) 

Definitions 2-7 

Deflection 25,  30 

measuring 96-97,  100 

Deflectometer 99,  106-107 

Deformation,  measuring.  .  .  .103,  107 

Density 54-58 

Diffuse-porous 44,  50 

Dogwood 22 

Drying 75-84 

effect  of 75-78,  138-139 

Dry  rot 69 

Durability 53,54,75 


PAGE 

Fibre-saturation  point 78 

Fibre  strain,  rate  of 92-93 

stress 15 

at  elastic  limit .  54,  62,  75,  123 

formula,  98,  102,  104,  107, 

114 

Fir,  Alpine,  16,  20,  27,  32,  40,  42,  57 

amabilis 16,  20,  27,  57 

Douglas,  13,  16,  20,  27,  32,  36, 

40,  42,  48,  55,  57,   124,   140, 

141,  142,  143,  144 

white,  9,  16,  20,  27,  32,  40,  42,  57 

Flexibility 5,  37 

Flexure 4,  12,  60 

Formula,  98,  102,  104,  107,  110,  114, 

123 

j   Frost  splits 62-64 

Fungi 59,  68-70,  75 


EARLY  WOOD 44,  82 

Ease  of  working,  factors  affecting, 

48,  50 

Ebony 22 

Elasticity,  modulus  of 6,  25,  89 

formula  for,  98,102,  104,  114,  123 
Elastic  limit 2,  5,  22,  62 

resilience 6 

formula  for 98,  102,  104 

Elm 8,  38,  44 

rock,  9,  13,  16,  20,  27,  32,  40,  42, 

56 

slippery 13,  27,  32,  40,  56 

white...  13,  20,  27,  32,  40,  42,  56 

Elongation 3,  7,  33 

Eucalyptus  globulus 54,  78,  82 

viminalis 54 

FACTOR  OF  SAFETY 29,  139 

Failures,  bending 33-37,  77,  78 

compression,  endwise,  12,  15-19, 

104 
cross-grained ....  10,  76,  77,  107 

shearing 19 

tension 8 

torsion  .  .  .  .  38,  39 


GRAIN,  cross 8,  59-61 

diagonal 60 

spiral 60 

Growth,  in  diameter 43,  44 

locality  of,  effect 70-73 

rate  of,  effect 43,-50,  72 

rings 44,  52 

measuring 95-96 

Gum 22,  44,  51 

red 27,  45,  56 

HACKBERRY,  9,  13,  16,  20,  27,  32,  42, 

51,  56 
Hardness,  2,  39-41,  54,  114-118,  133 

Heart  break 65 

shake 64 

Heart  wood 50-54,  58,  73,  75 

Heat,  effect  of 84-86 

Hemlock,  9,  13,  15,  16,  20,  22,  27,  32, 

40,  42,  44,  45,  56,  78 

western,    48,  59,    140,  141,   142, 

143,  144 

Hickory,  8,  22,  38,  40,  43,  44,  49,  51, 

53,  55,  59,  65,  66,  72,  124 

big  shellbark.  .13,  16,  20,  27,  56 

bitternut .  .     .  .  13,  16,  20,  27,  56 


INDEX 


163 


PAGE 

Hickory,  mockernut .  13, 16,  20,  27,  56 

nutmeg ...  13,  16,  20,  27,  56 

pignut,  6,  7,  13,  16,  20,  27,  48,  56 

shagbark 13,  16,  20,  27,  56 

water 13,  16,  20,  27,  56 

Hollow-horning 80 

Honey-combing 54,  80 

IMPACT 30-33,  110-114,  132 

Implement  woods,  testing 124 

Indentation 39,  117-118 

Injuries,  fungous 52,  59,  68-70 

insect 52,  66-67,  72 

marine  wood-borer 67-68 

parasitic  plant 70 

KEROSENE,  effect  of 87 

Knots 51,  52,  61-62,  89 

LARCH 8 

western,  36,  48,  64,  140,  141,  142, 

143 

Late  wood 44,  59,  82 

measuring 96 

relation  to  strength 47-49 

Limit  of  elasticity 5 

Limnoria 67 

Live  load 28 

Load,  application  of 29 

concentrated 28 

dead 28,  144 

immediate 28 

kinds  of 26 

live 28 

maximum 29 

permanent 28 

safe 29 

uniform 26 

Loading,  centre 97,  100 

static 29 

sudden 29 

third-point 97 

vibratory 29 

Locust 22 

black 13,  40,  44,  51 


PAGE 

Locust,  honey,  9,  13,  16,  20,  27,  32, 
40,  42,  56 

Log  of  tests,  97-98,  100-102, 103-104, 
107,  110,  114 

MACHINE  for  static  tests 90-92 

Maple 22,  44,  51 

red 13,20,27,32,40,42,  56 

silver 13 

sugar,  9,  13,  16,  20,  27,  32,  40, 
42,  56 

Marking  test  specimens,  94-95,  100, 
129-131 

Material  for  tests,  88-90,  94,  99-100, 

102, 106,107-110,113,115-116, 

117,   118,   119-120,    121,    122, 

123-125,  128-134,  135,  136 

Mechanical  properties,  definition  of ,  1 

factors  affecting 43-87 

Medullary  rays  (see  Rays) 

Mistletoe 70 

Modulus  of  elasticity 6,  25,  89 

formula,  98,   102,   104,  114, 
123 

of  rupture 26,  54,  62,  75 

formulae 98,  102 

speed-strength 94 

Moisture,  determination .  90,  133-134 

effect  of,  6,  8,  17,  33,  75-84,  138- 

139 

Mulberry .44,  51 

NATURAL  shape  and  size 2 

Neutral  axis 23 

plane 23,  33 

OAK,  15,  22,  43,  48,  49,  55,  60,  66,  71, 

84 

black 40 

bur 13 

live 22 

post,  9, 13, 16, 20, 27, 32, 40, 42, 56 

red,  9,  13,  16,  20,  27,  32,  40,  42, 

56,  124 

southern 52,  71 


164 


INDEX 


PAGE 

Oak,  swamp  white,  9,  16,  20,  27,  32, 

42,  56 

tanbark 56 

white,  13,  16,  20,  27,  32,  40,  42, 
54,  56,  72,  144 

yellow,  9,  13,  16,  20,  27,  32,  42,  56 
Osage  orange, 

13,  16,  27,  32,  40,  51,  56 
Oven-dry 57 

PACKING  BOXES,  testing 124 

Permanent  set 5 

Permeability 54 

Pitch  pockets 65 

Pith  rays  (see  Rays) 

Pine 44,45,55 

Cuban 71 

loblolly,  36,  48, 51, 54, 71, 85,  140, 
141,  142,  143 

lodgepole,  13,  16,  20,  27,  32,  40, 
42,  57 

longleaf,  6,  7,  8,  9,  13,  14,  16,  20, 
27,  32,  36,  40,  42,  46,  57,  59,  65, 
71,  78,  140,  141,  142,  143,  144 

northern  yellow 22 

Norway  (see  Red  pine) 
red,  9,  13,  16,  20,  27,  32,  36,  40, 
42,  48,  57,  140,  141,  142,  143, 
144 

shortleaf,  13,  27,  36,  48,  57,  71, 
140,  141,  142,  143,  144 

southern  yellow 22,  124 

sugar,  9,  13,  16,  20,  27,  32,  42,  57 

western  yellow,  9,  13,  16,  20,  27, 

32,  40,  42,  57 

white,  13,  15,  16,  20,  22,  27,  32, 
40,  42,  51,  57,  144 

Plane,  neutral 23,  33 

Plasticity 6 

Pliability 5,  38,  85 

Poplar 22,44 

yellow 44 

Pores 44 

Preservatives,  effect  of 86 


PAGE 

RAYS 60 

effect  on  compression  failure, 

17,  18 
shrinkage 81-82 

Redwood,  13,  16,  27,  36,  48,  57,  140, 
141,  142,  143,  144 

Resilience 2,  5,  49 

elastic 6 

formula?  for. 98,  102,  104,   114 

Resin,  effect  of 59 

pockets 65 

Rind-gall 65 

Ring,  annual 44 

growth 44,  52 

-shake 64-65 

-porous 44,  59 

Rot 68,  69 

Rupture,  modulus  of. .  .26,  54,  62,  75 
formula? 98,  102 

SAFE  LOAD 29 

Sap 73,74 

-stain 59 

-wood 50-54,  73,  74 

Sassafras 51 

Season  checks 61,  78-84 

of  cutting,  effect  of 73-75 

Seasoning 55,  74,  75-84 

Second-growth 49,  50 

Set 5 

Shake 64-66,  72 

cup 64 

heart 64 

ring 64 

star 64 

Shear 3,  19-22,  133 

across  the  grain 19,  21,  22 

along  the  grain   19,  21,    76,  94, 
107-110 

formulae 98,  102 

horizontal 24 

failure 35-37 

longitudinal 21,  24 

oblique 21,  22 

transverse.  .  23 


INDEX 


165 


PAGE 

Shear,  vertical 23 

Shearing  strength.  . 2,  19 

Shipping  dry 57 

Shipworms 67 

Shock 30,49 

Shortening 3,  33 

Shrinkage 54,  58,  74,  76,  78-82, 

135-137 

S-irons 83 

Site,  effect  on  wood ....  48,  49,  70-73 
Size  of  test  specimens,  effect  of, 

89-90,  138-139 

Sketching  test  specimens,  94-95,  100, 
102,  106 

Softwood 44,  60 

Span 25 

Specific  gravity 55,     135-137 

Speed  of  testing  machine 93-94 

-strength  modulus 94 

Sphceroma 67 

Spike-pulling  test 123 

Spiral 'grain 60 

Splintering  tension  failure 34 

Splitting 41,  60 

Spring  wood  (see  Early  wood) 
Spruce,  14,  15,  22,  44,  59,  75,  84,  144 
Engelmann,  13,  16,  20,  27,  32,  40, 
42,  57 

red 7,  13,27,57,  140,  141 

white 13,  27,  57,  140,  141 

Static  tests,  machine  for 90-92 

Steaming,  effect  of 85,  87 

Stiffness.  ...  1,  4,  5,  6,  25,  26,  62,  76 

Strain,  definition  of 2 

unit 3 

Stress,  compressive 3 

definition  of 2 

due  to  impact 31,  32 

external 2,  33 

internal 2 

shearing 3,  21 

tensile 3,  62 

torsional.  .  38 


PAGE 

Stress,    unit 3 

-strain  diagram 3,  97-98,  100 

Structural  timbers,  strength  of, 

138-144 

Summer  wood  (see  Late  wood) 
Sycamore,  9,  13,  16,  20,  27,    32,  42, 

57,  64 

TAMARACK,  9,  13,  16,  20,  27,  32,  36,40, 
42,  48,  57,  140,  141,  142,  143,  144 

Temperature,  effect  of 84-86 

Tensile  strength 1,  7,  23,  78 

parallel  to  grain 7,  8 

right  angles  to  grain 8 

Tension 7 

failures 34 

tests 118-122 

Teredo 67 

Tests,  impact 31-33 

timber 88-136 

Test  specimens,  size  of 89-90 

Timber  testing 88-136 

VEHICLE  woods,  testing 124 

Variability  of  wood 1,  2,  43 

WALNUT,  black 22 

common 22 

Water  content 55,  73,  74 

effect  of 6,8,  17,33,  75-84 

Wear,  resistance  to  (see  Abrasion) 

Weight,  relation  to    mechanical 
properties 54-55 

Willow 44 

black 13 

Work  (see  Resilience) 30,  54 

Working  plan 88,  127-137 


XYLOTRYA. 


(37 


YELLOW  POPLAR 44 

ZINC  CHLORIDE,  effect  of 87 


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